Just one more angle to look at...

What does it mean to you when he smokes?

Could it be possible that it bothers you not because of the legal or possible health stuff, but because it feels like he is not spending time with you?

If you are craving intimacy, or a closeness with your boyfriend, the pot might aggravate the situation because its just like when someone drinks-they are intoxicated, you are not, and so its hard to connect, if that makes any sense.

Do you feel unloved/abandoned/neglected when he smokes?
Do you get anxiety when he smokes?
just thoughts to help you explore some of the maybe unconscious things going on for you.

Your boyfriend likes to smoke, that could be anything from his way of coping to his hobby, either way, it is apart of who he is-he may eventually grow to resent you for trying to control that part of him.

Coming home finding him smoking-that might make you feel like he's not listening to you, or possibly even that he doesn't care.

He may care deeply about you, but also he has the right to be himself too, as you also have the right to yourself. What if he told you he didn't like your music, and you could only listen to your favorite music on the weekends? You might try and make it work for awhile, but eventually its going to aggravate you that not only does he not like your music/hobby, but that he is also no controlling something you love, that is a part of you. Its just not a good situation.

you're in a hard place, I hope you keep posting;
Take care,
-obj