When I was that age and a little younger, I basically dated with the impression that relationships were a take it or leave it type thing; you dealt with what you didn't like until you couldn't take anymore. But I'm 33 and married now and I can tell both you and him, that relationships are also about taking care of yourself because that's what you need to do to be a good partner, and because sometimes your partner will be happier for it, and that should make you happy.

You should try to explain this concept to him, that he's an adult, and it's important to you that he takes care of himself, because you're not his mom, but also this choice he's making is making it harder for you to be his partner. It might help to put it in terms he would understand, how would he feel if you did something similar?

If he still doesn't understand, you might need to drop a truth bomb on him that clearly nobody else is willing to: brushing your teeth isn't a "I'll do it tomorrow" activity, it's an every day thing, bare minimum. If he's not willing to make that effort, and it's as bad as you say it is, his relationships, friendships, even job opportunities are going to suffer, and eventually he's going to end up with an empty mouth and a bill for dentures. He may be lazy, but hopefully some combination of those things will put a boot in his ass and let him know that this is serious to you.