A couple of suggestions: try gliding, and put obstacles in his path.
Why gliding? Well, IMHO it is more fun (0-50kt in 5s, pre-solo aerobatics), many clubs will let you start learning at 12 and (if safe) go solo at 14, it is probably significantly less expensive than you imagine, and they will automatically learn how to act responsibly, maturely and safely in aircraft and on the field. I've a great deal of respect for all such young people at my club, my daughter included. See
British Gliding Association >> Find a Club >> UK Map .
Why obstacles? At about 10 or 11 my daughter announced that she wanted hens as pets (this was
long before they became fashionable!). I didn't particularly want them, so I kept on raising individual specific (i.e. not vague generic) valid reasons for not having them: noise, smell ,vermin, holidays etc. She listened to my concerns, understood them, did
good research on the net to come up with coop designs and
convincing reasons why my concerns were invalid. After 6/9 months of that I was pleased to let her have them, and she looked after them very well until she took herself to Aus at 18.