As Nessa says, only you can make the decision. You have lived with a substance abuser before so you have some idea of the pitfalls. I also suspect that your history will probably make you more resentful of the smoking than would otherwise have been the case. Only you know whether it is something you can live with or not. If you can't, if you will end up feeling angry and/or hurt a lot of the time then he has to stop or you have to go IMO. You can't go on with negative feelings all the time, none of you (incl your bf) will be happy and it's not fair on any of you, especially the kids.

Hugs and very best wishes to you. I hope that it all works out for you.