Tall 10yr old dilemma - full-suspension

Ones gonna be to small in a year, the other will be to big for a year. I feel I would go the 27.5 rout in a small and although he will probably not enjoy it as much for a bit he will adapt to it quickly.

The jump in handling (especially in the air) between a 24" hardtail and 27.5' full suspension is going to be a lot.

For what it's worth I've had the same dilema for my just turned 11 5ft giraffe :o)  I decided to go systematically up through the wheel sizes and so at 10 he was riding a 26' Chromag Minor Threat  (yeah he's very lucky... but he puts it through it's paces in Whistler, Squamish, Sun Peaks etc)
He's just moving up to a 27.5 Transition Scout this year.

I wouldn't focus too much on the actual wheel size but more on the traditional measurements of reach, stack and ETT.  There's quite a substantial variation between manufacturers and so some 26" bikes might fit better than a different 27.5. 

Bike geometry geeks website or similar is your friend here. A quick look shows that the The YT Jeffsy 26 has a reach of ~400 and and a ETT of 532 (this matters for making sure they aren't super stretched out like a roadie pedalling up hills) which is pretty identical to a my sons Minor Threat and I reckon he'd have been okay on it up to about 5ft1 - but as with adults the fit is quite dependant on how much leg vs torso any child has...  

The Extra Small Scout is similar in sizing with slightly more reach at 410 and on the bigger 27.5 wheel.   Rocky Mountain Reapers are a much shorter reach which gives a more upright pedalling position.

I've tried to keep the jumps in reach to about 30mm between frames... At the rate my son grows this seems to give me two summers out of one bike - it starts off a bit big and finishes a bit small.  I do things like switch stems and handle bars after a single season as well to try and keep the fit working.  E.g year one is 35mm stem and a 680mm wide handle bar on the Minor Threat and year 2 was a 45mm stem and a 700mm wide handle bar. 

A final point is that adult smalls in 27.5 are getting increasingly rare so you might be restricted to kids bikes unless you get lucky - even the used market is challenging.  They're quite often mullet (29 at the front) which I just didn't think was a good idea as the weight of the front wheel would just be very heavy when jumping etc.   Be very wary of older used bikes as their geometry is potentially very different to a modern bike and depending on what your sons 24" bike is that might be a big step backwards.  My son has never ridden anything with a head angle steeper than 65 degrees so an old small bike would have had him over the bars tout suite.

Gratuitous proud parent photo from when he was 9... :o)

Got to nail that landing on a hardtail!!

Commencal Clash XS is an option. It's a 27.5/26 mullet designed for tweens. It's fairly close to an adult-sized bike but not quite. My friend's son has one and it's awesome. 

Long time reader and first time poster. I don't have kids, but do work in the industry and love seeing young kids falling in love with mountain bikes. A couple thoughts from sizing bikes for a long time in bike shops. Kids tend to have much longer limbs than a full grown adult at the same size. What this leads to is being able to "stretch the rider or shrink" the size charts that most brands post. Example below with the 27.5 Giant Trance X. Super rad shredder bike, especially for a lighter rider, but the size chart suggestion starts at 5'4''. This is a guess without seeing your son sit on the bike, but there's a very good chance he would actually fit this just fine.

There is also the idea behind Giant's sister brand Liv. Yes, I know a girls bike doesn't always fly with a young boy but hear me out. Liv's geometry is based around women traditionally having longer legs and shorter torsos than the same height male. Top tubes and reach are shorter, and stack is usually taller than the Giant counter part. Shock tunes are also lighter per each size, and again based on averages here, women are lighter weight than men per the same height. This is almost exactly true with young males: long limbs and lighter than full grown adults. The Intrigue X mix wheel comes to mind at 140mm Rear and 150mm fork.

Last point: 'Expensive' small size adult bikes are much easier to resell in my experience, than expensive youth bikes. The second hand market is much smaller for little shredder bikes.

27.5 front and rear Trance X on sale https://www.giant-bicycles.com/us/trance-x-3-2022

Good luck with the search. There's so many cool bikes out there!