How do you put your foot down?

My boyfriend wants to try to smoke cigarettes and I am so against it because it's not good for his health. I'm worried that once he tries it he won't stop. I have begged him over and over to not even consider it but he doesn't seem to want to listen. I don't want to break it off because of something like that. I think about the long run though the effects it could have on his health. & I don't want my kids to loose their dad. And I don't want to lose my partner. I don't know what to do to make him not want to. I'm conflicted and overwhelmed.