I play the pennywhistle (tin whistle, as it's called in my part of the world) and have taught it to a few groups of kids. They always love it, and it's always a challenge on multiple levels, both mental and physical. I hope for the sake of your ears your son has the breath control thing down!
If you haven't already, you and your son should check out
http://www.thesession.org. It's got thousands of tunes, plus listings for local trad music sessions if your son wants to find some people to play with (one session I used to play in had a 12-year-old fiddler who was perfectly well-accepted as one of the group).
