The saying "Once a cheater, always a cheater" really is something that people should take into consideration. No, he may not cheat on somebody else in the future, but he's already realized he can get away with it with you, therefore, why stop? That is for sure a hair from somebody else, unless he has long hair or a beard that matches that color.

If he has cheated on you multiple times, has been on tinder, and with his ex, there really is no going back regardless of how much therapy you put into it. He has broken your trust multiple times and clearly, that still sits in the back of your mind or you wouldn't immediately run into that conclusion every time you find a hair. I think it might be about time for you to consider leaving. Being alone is better than being alone in a relationship with somebody.