I tend to agree with Fuzzy on this one...honestly, you're doing a really fine job of trying to overcome your issues and being honest that this is at least partially YOUR issue as well as his. I admire that in your post.
From the outside looking in, and just having been around the block a few times - if you already know he has a propensity towards addiction, chewing tobacco is probably one of the better ones to have. He's not giving people cancer from his S.-hand smoke, he's not driving drunk...yeah it's disgusting but if he's keeping it away from you I doubt you're tasting it on him or finding spit cups everywhere (EW! And yes, I've been through that). So I vote for giving the guy a break.
Honestly, he seems like a very stand up, very sweet guy. I feel bad for him because it is obvious he really does have good intentions. I would try really hard if I were you, not to nag him about this, and to just tell him you love him and are here for him. And thank him for keeping it away from you and the kids. Obviously he knows you don't accept it or condone it. He knows it can cause cancer, too. Maybe if you talk to him about it you can agree that he will try to quit again when things settle down (you mentioned stress he's dealing with currently.) It may end up being a deal breaker for you, but that is your choice. I think realistically, lots of people have lots of nasty habits, many, worse than this. And they get along fine in the world, somehow. Only you can decide what you can and can't live with, however. Everyone has their own limits. Good luck!
(ETA - As you say, he chewed tobacco when you met him. So you have to keep that in mind. He probably only quit because YOU wanted him to, to begin with. That will never keep someone away from something they enjoy and are addicted to. So I definitely vote give him a break. I also agree with Christy Lee in that he is not an alcoholic - however, tobacco IS addictive and if he has to have it to cope with stress, then yeah, he is probably addicted. Just don't go overboard, seeing addictions all over the place where there aren't any.)