I know it can feel like there are only young moms on here - I have a 12 and 13 year old, and they are quickly coming upon the older kid problems. I can see them already creeping into our lives...
My nephew is 16, and does smoke pot. He stayed with us for a while this summer, PLUS I have *tried* to deal with this news from Texas (when he was in NC) while his parents were on vacation in LA! Be grateful you can actually see and touch your son. Imagine if he was half way across the country!
What I have learned with him.... They are at an age where they will leave the house and be forced to make their own decisions. Some decisions will be great - others will be so very stupid. All you can really do is talk WITH him (make sure to engage him in the converstation), tell him where you stand on the subject & what you expect from him. Let him know what will happen if he does X, Y, or Z - then DO it (like you already are).
I know it sucks. It is scary watching them walk out the front door KNOWING they are supposed to be doing something that is just fun and safe (like school), but hoping deep down they are really going to do that activity and not something they (and you) will regret.
To answer your question, I think you are already doing all that you can do. You are sticking to your guns. Just make sure you both can have a real conversation about this when you both cool down.
Good luck.