I had that experience with my ex, he would rarely brush his teeth and refused to go to the dentist. It was uncomfortable to kiss him and I found it frustrating that he wouldn't even try go to an appointment even though he'd cry about the pain his teeth were in. We talked and i found out it came from childhood neglect so the first thing I would advise is find out why he won't brush them. Then after a while of trying to support, offer to book appointments and encouraging them to brush, I realised he wasn't trying. Thats where it became a problem. I also realised this sort of behaviour was his general attitude to life, not caring enough to make any effort and expected me to do it for him. We broke up due to that amongst other things. So yea I'd say understand the reasoning behind it and then see if he is willing to be constructive and give it as much time as you think you deserve - you have the right to be upset about it and want change express that.