What do you do when the boy you love has a girlfriend but he would go out with you if they ended?

Wait until the relationship ends. If he really does like you, then he will end the relationship. Right now, he is sending a message: "I like my girlfriend more than you because I am with her now." You should have respect for yourself and stop liking him because obviously you are second choice only and you don't want to be someone's second choice. You dont' want to always be the girl on the side that he messes with when he deosn't have someone.