Everywhere I look hyperskeptic is giving really awesome advice. Not much I can add so I'll just say what I'd do in your position, which is that I'd need him to talk to his girlfriend clearly about poly, have discussions with her about safe sex, and other boundaries so both of them knows just what is going on, before being intimate with him again.
She should have the right to know where she stands so she can choose to leave the relationships if monogamy is important to her. She should have the right to ask for a DADT situation if she prefers that (though I don't really see how that would work when you are all friends). She should get to decide if she is willing to be in a mono-poly relationship or wants to explore poly herself, I'm not clear that they've really discussed what all this means enough for her to even know what options there are or what he wants out of a relationship.
You deserve to feel good about caring about somebody and falling in love with them, so I do think that trying to keep things above board is really going to provide you with that. Really as hyperskeptic says, stop assuming and start asking, because if you end up in a relationship with him, you'd want to know all those same things I mentioned in the previous paragraph about other relationships he's having wouldn't you?
Happiness will never come to those who fail to appreciate what they already have.