Depends on his height, and what he intends to do with it. Is he likely to want to ride loads of XC and go exploring bridle ways and so on, or is he more likely to want to rag it round skills parks and trail centres? Is he going to be sedate and glued to the ground or is he going to go looking for places to shred and jump and so on?
The standard response will be a hardtail. If you look around you'll find 14 and 16 inch Hardtails aplenty. If he wants to ride competitively, racing and so on, look for a hardtail XC oriented bike. Otherwise look for something more trail oriented, or dirt jump style. Something like a Scott Voltage, NS Surge, DMR Reptoid etc.. Hell, he may even get on with a big BMX.