Wow! Thanks for all the replies.

Several of you suggested sharing a spliff/bong with him.... personally I would feel uncomfortable doing that. I fear he would see it as giving him the green light to smoke whenever he liked.

As I've said before, I've tried the tough approach, to no avail. So maybe now is the time for compromise and bargining. Maybe turn a blind eye at the weekends as long as his schoolwork is done during the week.

What I really would like him to do is, lay off the cannabis altogether, untill he's able to take a more responsible attitude towards it and use it purely recreationally. Maybe that's just a pipe dream.