You need to get a paternity test. If he's sure that's it's not his then he should have no problem with it. Even if it's not his, this doesn't necessarily mean that he hasn't been cheating, only he knows the truth about that. The fact is that you'll never actually know the whole truth, you just have to decide whether or not you are ok with what's he's done and it sounds like you are. So the only thing you really need to know is whether or not the baby is his.