What do I do with my cheating boyfriend?

Ive been dating him for almost 7 months and I know he's cheated before. Currently I'm out of town for the summer, so I haven't seen him in a few weeks. Recently he was out with a few friends, got drunk(very drunk) and slept in a girls bed. He says the girl was trying to do things other than sleep, but that he's almost positive that nothing happened. The problem being that he doesn't remember about 50% of that night, so he's not really sure. I'm not sure what to think. I'm mad at him, but I still love him, but now I'm worried about what realy happened. What does it mean if they did have sex? both for our relationship and health? I know I'm not going to leave him, but now I'm not sure I trust him with his friends! What do I do?