I don't know that it's such a bad thing for your husband to be looking into the future. If he's sure he won't be comfortable with you having sex or seriously dating someone, wouldn't you like to know that now?
Try to think of yourself as discussing a possibility with him, rather than trying to convince him to do something. Remember to listen. It will be obvious to him if you don't really listen to him, acknowledge his feelings and address his concerns. You don't want to start your poly life in a place where he feels unheard, unimportant or powerless.
It seems like you think he should agree to being poly because of his past cheating. To you, only the lying hurt. He may have experienced the worst guilt about the sex rather than the lying. He cheated when he shouldn't have, but that doesn't mean he ever thought non monogamy was a part of the life he wants. He might be more like the guy who has trouble avoiding the dessert cart but knows he doesn't want to be unhealthy. Sure, he may eat three pieces if chocolate cake, but that doesn't mean he is willing to commit to exploring an unhealthy lifestyle. The only way you can find out is if you listen to him.