@IoanSaid XS Trek X-Caliber, 27.5 wheel, if he's tall enough and got enough standover. Short stem and he might be good to go. My son kicked serious tail racing on the one I built up for him. Bought the frameset then threw on some decent parts. Truly a great bike. Another kid in our area is now racing on it after he outgrew it. Handled XC racing and technical black diamond trails in Moab, UT, Arkansas and the Michigan UP easily.
@idropyou not enough standover.
Also he's got long legs but a short body so I did adapt his current bike from 70mm stem to 50mm.