There's a father and son who sometimes ride with our group. I think the son just turned 15... he's gone through three bikes in 18 months due to growth.

I think you're best bet is to find a good, used 54 cm road bike. It can "grow" with him for a bit by raising the seat and getting a longer stem.

If you end up having to buy a larger bike (and he's still under 17), you should put the shorter stem back on the used bike, and save the longer stem for the newer bike... you'll probably need to size that one up some as well.

With luck, two used bikes will get you through his growth years, and he can buy his "grown up" bike when he's 18 or 19.

Good luck!