Youre kid's almost old enough to make his own decisions.
IMHO you should just talk to him, tell him you smoke; tell him the good and the bad about weed; tell him that the legality is not so much of an issue as the morality (meaning that if something is illegal, but it isnt morally wrong, one should give themselves the right to decide whether they would like to do it.)
Then after all of this, if he seems open minded, offer to smoke with him. He's not really an adolescent any more. My thinking is that he's most likely going to try it sometime in his life. Why not have that time be with his father in a safe secure environment, instead of out somewhere where he could get caught.

edit: Hey happiestmferoutthere, that thing is your sig reminded me of this thing my friend always says.
"The beatings will continue until moral improves."
idk if its a quote or what, i just think its funny.