A fast twist .223/.223AI has so much going for it...youth or adult. Aside from rimfire it's hard to beat it for cost to feed, ammo availability, good component choices, barrel life, low noise/blast/recoil, platform options, etc. All those things make it possible to shoot more...and nothing beats shooting more. I like the AI option and no downside to be able to shoot plain jane .223 is a plus....but even standard .223 twisted fast is great.
Considering growing with him...it's not just fit of rifle. If he likes to shoot/hunt, he'll never grow out of the usefulness of a .223.
Considering fit of rifle, I'm not a big chassis person but after using one with my 3 kids..and myself...the MDT LSS with an adjustable AR stock has a lot of flexibility in LOP. One thing to keep in mind for a kid, that often gets skipped over, is cheek weld. Kids often get a stock with a shorten LOP but no thought given to drop on comb. It's an easy fix to raise it with bear tooth or stick on neoprene pads for ar stocks.
