I smoke casually, I go without it for months then have a couple here and there when I feel like it. I recently was out with a girl who scolded me for doing so (she has never smoked). It was very annoying, I understand and respect the non-smoker opinion and don't smoke in peoples cars, houses whatever like I own the place. Point is the constant nagging at it will NOT help anything, and probably bring your guy a lot of stress, tension and resentment toward you.

He expressed interest in quitting and this is something he has to do on his own at a time he wants. In the mean time you will have to live with it and accept, or move on.

You can bring up the fact you want him to quit and this is a deal breaker. Unless you're prepared to move on immediately if he says he is not willing to quit at this time (the deal breaker) you're probably just nitpicking at a small issues normal couples work out daily.