I have always subscribed to the belief that once a cheater always a cheater. I've been cheated on before and felt like an idiot for no seeing the signs. Had I realized sooner I would have ended it, she was a ***** 90% of the time anyway so angry.
You def have to talk to him. If you don't its going to consume you. You'll think abt it every time he's alone. Are they texts or emails? Could it be like a porn chat thing, then it's just a fantasy at least. But you def need to confront him regardless. How can you trust him if you don't.