I'm surprised by the preponderance of zero-tolerance anti-weed people in this thread. I completely agree that OP's preference for a non-smoking boyfriend is a legitimate desire and that it's up to her personally whether it would be something she would consider leaving the relationship over, but come on guys, weed is just not that big a deal. A ton of people smoke it regularly, and plenty of heavy users (even daily smokers) are perfectly normal, well-adjusted, happy and productive members of society. The major downside of the stuff is that it's illegal, which has nothing to do with the drug per se and everything to do with bad drug policy.
OP, your boyfriend is not likely to actually quit smoking if it's something he likes doing. He's more likely to just start resenting you for trying to change him and forcing him into hiding it. FYI that's not because weed is an evil, super-addictive substance, it's because prohibitions and ultimatums are not characteristic of open, respectful communication and compromise in relationships.
Overall, though, it sounds like maybe you guys have some differences that are only going to become more and more obvious and important as time goes by, and you should give serious thought to ending the relationship and sparing both of you some future suffering.