Thanks for all your replies.

I've tried the coming down hard routine with him before, all it did was cause friction, arguments and resentment.

I have talked to him at length about the reasons why he smokes it. From what I can gather it's because all his mates do and he enjoys doing it.

Short of locking him in his room for 24hrs there doesn't seem much I can do about it, but try to keep the lines of communication open and hope he's sensible and responsible about it. (Now he's been caught red-handed and suffered the consequences, he might just come to his senses.)

By the way I'm his Mother not his Father.