Alternatively, instead of asking him to give it up, you might consider whether this is a thing in your relationship you can tolerate.
If he refuses, in the end, is it something you can live with?

I've been in the situation, though the weed wasn't the thing that bothered me, but his actual cigarettes (let me tell you, kissing someone who's smoked weed is like a thousand times better than kissing someone who's smoked tobacco in any form), and continually pressing your significant other to give up something they enjoy makes for a stressful relationship full of resentment.