I would even try him out on an electric. They're usually heavier, so make sure he wears a strap, and start him out sitting down. Get a small amplifier too.
The reason I mention it: Electrics are easier to fret than acoustics (even nylon string).
Start out with some easy power chord songs, and that'll help his confidence when he can actually play a song in no time.
check out web reviews like this one below:
http://www.electric-guitars-guide.com/child-electric-guitars.html
Keep up the good work at encouragement! :p