What do you think he would do if you told him you want him to leave her alone for good and commit to you? Being honest with him, telling him what you need in order to stay, will bring it to a head. You have every right to give him an ultimatum. But, even if he agrees, he is so involved with her. Do you really think he can let go? It's hard to put yourself in the position of being worried that he is really leaving her alone and trusting him.