If he likes athletic stuff, my guess is that he will be riding that road bike a lot. I know I rode the heck out of mine when I was that age. I think if you got him a decent bike where the seatpost only had a 2" extension from the frame that he could ride that bike for many years, just by raising the saddle and putting on longer stems.

OTOH, it's possible to go like stink on a MTB with the right tires and position. It's totally not necessary to be sitting upright on a MTB. Standard road position should be possible. That's faster on the trails anyway. You want 1" slicks at 100 lbs. for the road.