I say get over it and let him smoke some pot if he really wants too. I smoke pot when I can....and it is harmless. It has no addictive components, it doesn't make you a junkie. Pot actually helps some people...for production purposes. Ending a relationship solely on pot is a mistake and will be regretted later. Allow him to find his own boat...if he begins acting risky then there's really nothing that could be done...if he starts using his drug of choice again, then that will be a good enough reason to split your relationship.

This is all my opinion really. I could be wrong according to your circumstances...but I'm just saying.