Northwestraven,
He knows full well there's a bad smell (although it might not necessarily be caused by his teeth, for some, it can be an issue with their stomach), but that's by the by, he knows a smell emanates from his mouth and mouthwash does not mask it. Personally, for me, this would be enough to decline to see someone, no matter how lovely they are. And really, if he was lovely, he wouldn't be kissing anyone given the bad smell.
I don't think mentioning it would be embarrassing him at all. He's an adult, one who is paying for sex, and a man who knows kissing him is not pleasant. He would get a better service if he dealt with the issue (and this would be the best way to raise the topic with him).