Hey, wait - what's wrong with being 4'11"????? I'm 35 and am 4'9"!!!!!!

I'm sorry, I was just trying to lighten the mood a bit. If he has yearly checkups (which kids should still be getting at this age) and your doc hasn't been concerned, then I wouldn't be concerned yet. But if you're worried, go see your doc or call and talk to your doc. They may be able to give you some advice.
Also just tell him this - tell him that you know a person on the DIS whose grandpa was 4'11" and his wife, the grandma, was 5'11"!!!! Maybe he'll at least get a good laugh out of it. (and yes, that is true)