You are a very, very smart woman...have to say that straight up. And also, I've never heard this sensitive subject talked about on any board so I believe others would be interested. You are 100% correct by describing this as a dealbreaker. I, too, am big on dental hygiene and it's one of my biggies when I meet a woman. He will always have this problem...trust me on this...this is who he is, he's comfortable with bad teeth, and no amount of coaxing him will help. I would end the relationship as soon as you can, cry your tears, be sad...but be better coming out of it.
Bad dental health now will only get worse with age and further neglect which is sure to come. You will be a constant source of ridicule and gossip, which we all agree shouldn't happen...but it does and you know it. REd flags are there for a reason...act upon them. You cannot change adult behavior. Many a woman has tried and failed, only to waste away her best years on a hope and dream that he will change, oh I CAN MAKE HIM SEE THE LIGHT. Odds are, you cannot. Run for the hills. Good teeth are important and I would not settle for less than what I want in this life. You'll thank me years from now for giving it to you straight and honest.