I think you should tell him. I'm sure you would want to know if your girlfriend were cheating on you, I sure as hell would like to know if someone were cheating on me. If someone I didn't know came up to me and told me my bf was cheating on me, I might not believe it at first, I might even get a little angry at the person, but it would make me be a little more careful, and when I found out it was true, I would be very grateful to the one who told me.
I'm not sure if that made any sense at all....