You should probably go find out if this is true. Don't rely on other people telling you too much unless you trust those people. But try to find out the truth.
If things aren't like that, then that's great. But if it turns out that he does, then confront him about it and you should just break up with him (unless he has some sort of explanation but it's doubtful). Whether he denies it or admits it, if you find out that that's actually true, your relationship would no longer be healthy. Someone who cheats on you once is more likely to cheat on you again. You don't want to be with someone like that.