Don’t do anything, you have to respect the relationship he is in. Doing anything to compromise his current relationship reflects badly on you and the potential relationship between you and him. If he decides to end his relationship, that is his business and at that point you can step in, but for now, you just have to wait it out. If the roles were reversed, you would not want another guy/girl interfering with your relationship, and you owe that same respect to his girlfriend. Hang in there.