IMO your step son is a little pussy, no offense man but im only 18 and i know my dad smokes weed and i have known since the age of about 14 and still kept my mouth shut and if i knew it would jepordize his relationship i definitly wouldnt say anything. i bet when he goes to his friends houses he smells marijuana too. I dont know where you are from but at my school they did some kind of annonymous survey and it proved 85% of students smoked weed and this shouldnt be too much of a shock to him. if i were you i wouldnt share anything with him and i wouldnt try to bond either but thats only my opinion and it doesnt matter what i say because i am not a parent or gaurdian of any kind.