How tall is he? Is he still growing?

I'm built like a small teenager (5 ft. 3 in). and find I can comfortably ride most MTBs with smaller frames (15" or smaller). These can be found used on Craigslist with a bit of searching. As often mentioned here, many older hardtail MTBs have eyelets on the forks and dropouts for racks, and make good touring bikes.

If he's still growing, a used MTB also means you've invested less money on a bike that he'll soon outgrow.