Get him a smaller adult size with a compact geometry so that as he grows the bike can be adapted to his size. Compact geometry typically allows great range of expansion by way of the seat post.

My son's bike was stolen about the same time he was your son's age. He was heartbroken and I was determined that the next bike I bought, wouldn't be as hard on the wallet as replacing this one.

I bought a Motobecane 650HT from bikesdirect. See if it doesn't look like a good bike for your son. Save up to 60% off Mountain Bikes - MTB - Motobecane Hydraulic Disc Brake Mountainbikes

BTW the current price is $100 less than I paid June 2013.