I just started seeing this guy in his late twenties who I met on a dating site.
There’s a lot of truth to that and it’s worth weighing in this situation.
On one hand, it hasn’t really been long enough to expect a guy to give up what’s clearly a major part of his lifestyle for you.
On the other, a couple of months is still a couple of months of your life, and before you invest more time in him, you should know if he’s willing to invest in you.
Needless to say, you must tell him, not gently, but firmly, that his smoking bothers you. Maybe he’ll make a real effort to smoke less, which regardless of the result might be enough.
Maybe he won’t make any effort, which would be a sign unto itself. From here, you could try the ultimatum route—“It’s me or the marijuana!