Beginner advice

Oh, this is my specialty. I make data sheets and written guides about buying a bike.

Your best bet is finding a Giant Fathom, Trek Roscoe, or Specialized Fuse secondhand. I've even seen that base Fuse with the coil for under 1k on their website. If you're REALLY struggling to find anything you can always fall back on the Poseidon Norton as a bottom of the barrel stock build that you can basically upgrade frame up long term. The wheels and drivetrain are more solid than I thought however so yeah you're basically just looking at fork, brakes, dropper and contact points (tires pedals grips saddle).

Full suspension is an option secondhand at 1k but be wary. LOTS of cut corners to make FS viable such as things like the Giant Stance. And for an average beginner it's not bad but for a kid wanting to push and punish the bike that tiny shock driving a single pivot 'flex stay' frame on to a skewer axle is suboptimal. Luckily the new one has a thru axle, so look out for that. Probably not what you're looking for, but just wanted to clarify if you see stuff like Giant Stance under 1k (which is common)... It can be a monkeys paw wish in the end.

Here is a data sheet of 'min-maxed' bike values at MSRP (keep in mind MANY bikes are on sale right now):

And here is a general written guide on buying used bikes and modern mountain bikes and 'what to avoid': https://drive.google.com/file/d/1i8O5Ape_RKVVDyQAJsF5_ATCjDB_sWMT/view 

The cheapest hardtail that's truly worth buying is the Trek Roscoe 6. GREAT bike with everything you need. Dropper tubeless 1x etc.

But if you don't feel the need for an air fork the Cannondale is 100 cheaper and if you can find it through an REI that's a great place to purchase. Though, YMMV on service. I would probably still go to a normal bike shop for service. REI is just great for warranty/returns.

Here's a comparison of the relevant bikes using Vital's built-in tool: https://www.vitalmtb.com/product/compare/26392,38206,44266,24805 

For around a grand i would probably look at a used trail bike rather than new. You could get a new hard tail for around that but i think it would be a better use of money to look for a used full suspension cause it would give him more to grow into and if you go to any colorado bike parks you might want full suspension. I dont know whats on marketplace or pinkbike in your area but maybe a used specialized status 140 or something like that. For his height he might be more comfortable on mullet or full 27.5 rather than full 29.  

It sounds like you two are aiming for a suspension bike for the trails and that's fine. But there's a lot of merit to lots of hard tail riding to really learn bike skills. So keep an eye out for a clapped out dirt jump bike for riding street and playing around with dirt jumps. And it's lots of fun bombing short singletracks.There are lots available for less than $1000 new which should be dirt cheap used. 

I really don’t think a used bike is a good way to go unless you guys know what you’re looking at. Used bikes can be a great deal but also require knowledge to fix things. And in that price range and that type of bike there will be a lot that maybe need wheel truing (or replacement), tires, brake pads, drivetrain, etc. sure you can find good bikes, you can also find overpriced, clapped out deathtraps and spit shined turds.

just get a new bike, and then everything will work and you got a shop helping you out if needed.  Im of the unpopular opinion that EVERYONE should start on a hardtail and flat pedals. You learn so many skills you otherwise won’t. Also it’s just one less thing to maintain for the time being. 

a more succinct way to say this is looking for lightly used 'last year model' hardtails is much preferrable to buying an old 26 inch free ride bike that is not suitable and maybe not safe without the right knowledge just because its 950 bucks.

ultimately i dont think anybody has been pushing them towards that, though we do see many new members get swayed by big suspension bikes from 20 years ago for the cool factor. a lil part of me wants to buy every clapped out kona stinky i can find, but i know im not gonna do anything with  it.

That said, 1k is truly on the cusp of an okay bike to a great bike... So it's a decent place to go secondhand if it means getting a fully featured hardtail over one that is using a coil fork, or rigid seat post, or wire bead tires, or 2x drivetrain, etc.

Trek Roscoe 6 seems like a good buy once, cry once option. as opposed to saving 100-200 bucks on a coil fork Cannondale. So I agree with you on the go hardtail and go new instead of used sentiment. But if 1k was a hard cap on their budget, a Poseidon or Polygon are pretty iffy bikes. Only cool thing I saw was that base Fuse for 1k from Speshy

Well yes and no, it's more so about your long term goals. You definitely don't want a growing child who progresses quickly to be riding on a 100mm coil hardtail with skewer axles and stuff. But the reality is most of us started out doing exactly that. Punishing a cheap hardtail until we no longer could. The good news is you don't have to! I realize many of the options I highlighted may be out of stock due to popularity... Lemme try to find some options that are actually in stock for a medium-ish size.

Cheapest bike I'd get (size large black): https://www.poseidonbike.com/products/the-norton?variant=43854478639332

A smaller reach size large better for XC riding: https://www.bikesonline.com/xc-899~8488187 

Similar spec Trek: https://www.trekbikes.com/us/en_US/bikes/mountain-bikes/cross-country-mountain-bikes/marlin/marlin-5-gen-3/p/40773/?colorCode=black 

Slightly on sale Trek: https://www.trekbikes.com/us/en_US/bikes/mountain-bikes/cross-country-mountain-bikes/marlin/marlin-7-gen-3/p/36967/?colorCode=grey 

Similar Specialized: https://www.specialized.com/us/en/rockhopper-sport/p/221599?color=366668-221599&searchText=91824-6603 

Bottom of the barrel Giant: https://www.giant-bicycles.com/us/talon-4-2022 

Great Norco that can be delivered to a nearby bike store: https://www.norco.com/bikes/mountain/trail/fluid-ht/2023-fluid-ht-2/ 

Ultimately, there is always a bike that is 'justifiable' at almost all price points... But I like to think of it as if you're already prepared to spend 700 or 1000... It's likely worth the investment to spend 1100-1200 instead to get a bike with NO cut corners. That said, I bought that Poseidon for 700 almost as an experiment and I've been having a blast on it! It goes to show you don't need the perfect bike to have fun. The good thing is it has every standard, such as boost axles, to be upgraded to be as good as a ~2k hardtail (my long term plan).

But even the basic skewer axle, straight steerer, 100mm coil bikes are a solid ride. I'd say the budget should be about 700-1200 and you can start simple with a Poseidon or luck out on a used Stumpjumper full suspension. All viable options.

Can get a stump jumper evo s2 frame and marzocchi z1 fork for 1300 new thanks to 50 plus percent off clearance deals.  Look for used/clearance parts from bike shops and classifieds and you can probably get rest of bike built up for 1500 total.  If he doesn’t outgrow it the bits you don’t generally replace he will have very top of the line equipment and regardless he will replace the consumable parts of the bike down the road.  If he does outgrow it you will not have an issue recouping the majority of the investment and putting it towards a larger bike down the road.

of course their is also lots of great used deals out there.  Especially if you will drive to co.  But I think you will struggle to find a complete bike in your price range new (even on clearance) that will have great resale value for a growing kid.

I’ll give more general advice. It’s definitely a good option to buy a new whole bike, ideally a full suspension in the sale from a popular mainstream brand like Trek or Specialized. Although a hardtail for the same price with better parts may be more suitable if he’s happy to ride one, this just depends on how smooth the trails he rides are when he’s sat down climbing or it’s flat. He’ll be stood up going downhill, so a hardtail is fine there and will improve his bike control. These will have good enough parts and if your son gains an interest in mountain biking he’ll then find out which parts are good by chatting to people and will know what to look out for second hand, so can upgrade his bike down the line. Often parts by really good more niche brands go for 30% off new like Reverse carbon bars. This leaves him in a situation where if he’s really into mountain biking in a few years time, he’ll have a bike with a selection of well chosen upgraded parts (bought second hand) and can get a summer job to buy a current pro level second hand full suspension frame for $1.5k to $2k. He’ll then be in the position experienced mountain bikers are in where they buy a frame and choose all the individual parts. Doing things this way also allows him to move his bike towards the style of riding he’s most into, like lighter parts for fast cross-country style trail riding or heavier stronger parts if he enjoys going downhill more and starts doing jumps or drops. Also going for a bike which has a reach a little too long for him now (i.e. one size larger) will mean the bike is more stable initially and he’ll have to put in more lean effort and body movement to corner it, then as he grows the bike relatively will be smaller and easier for him to manoeuvre around. Essentially what you want now is a bike with the highest resale value and then a roadmap can be naturally developed for improving it with second hand parts in the direction he chooses as his riding style develops, so that the value of the bike he ends up with is the best for the money spent, with the original parts sold off to partly cover the cost of the upgrades. The first upgrades I’d recommend are a shorter second hand stem 40mm or 50mm in length if the one on the bike is longer and a pair of current or recent model second hand DMR V12 flat pedals.

The Specialized Fuse would be good enough for him. The X-Fusion forks are less popular than the more expensive brands, but they actually manufacture the forks for some of those other brands, so they know what they’re doing. All the other parts are good enough. He’ll just need the DMR V12 pedals I mentioned above, as the specialised pedals don’t have any metal pins for grip, lots of brands now make plastic pedals with metal pins which are good and more affordable than their much more expensive metal pedals. Also it would be worth making the tyres tubeless, where you take the inner tube out and replace it with special liquid latex that automatically seals up any punctures from thorns etc. It would be best to get a bike shop to do this first. Having tubeless tyres also means you can run lower tyre pressures to get more suspension from the tyres. Once you get the bike loosen the handle bars in the stem and adjust how much the handle bars are rolled forward or back, to whatever he prefers. Often under the stem there are a few spacers, so these can be moved above or below the stem to adjust the height of the handle bars. The saddle can be moved forward and back on its rails as well. The forks will have a recommended pressure and settings for his weight in the manual, so start with those. 

You are looking at the right 3 bikes. Right now is a buyers market for outdoor goods, not a sellers market, meaning that relatively speaking, a new bike is better value if you can afford it. Many brands are projecting shortages next year, so if you need to offload it, doing so next year will be much more favorable.

If he is going to hit 5'10" in a year and you want him to be on the same bike then I'd go large. If he were the oldest and you had a medium kid coming along to hand off to then that changes the calculus. But I'd plan for the future, and get him a slightly-too-large bike now. Ultimately your call, but that's how I see it.

Agreed. It's always better to have a bike that's too small vs. too big. Too small is an occasional nuisance, too big is a constant confidence killer.

Today's MD is yesterday's LG, so he'll be just fine. At 6ft tall I'm pretty sure I was riding what would be considered a SM today back when I was younger and did just fine sending sketchy stuff.

And to echo statements made above: new bikes are on fire sale right now, with warranties. This is definitely the way to go. In your neck of the woods I highly recommend dealing with Erik's Bikes. They're the largest dealer, made up of core, knowledgeable riders in the Midwest.

I’m 5 8-5 9 

I prefer lg frames for bike park and purpose made gravity trails and prefer medium for trail riding.  Lg frames feel amazing to pilot but get cumbersome on anything tight or flat.