I'm not one of those "what he doesn't know wont hurt him" type of people, I'm also not confrontational, and dislike confrontation if I can avoid it, so this situation would be difficult but i would find a way to bring out the truth, because what you don't know can hurt you, it can hurt you if you find out it's been going on for years, or decades it makes you feel stupid, it can hurt you because of diseases, and if he ever does find out, it can hurt him finding out you knew already.