I think you should be very honest with him. Tell him you are serious about making things work, but you are afraid that he will never leave his ex and make a commitment to you. Tell him that if that's never going to happen, he needs to tell you because you are both wasting your time. Maybe ask him what it is that she is providing for him. Why can't he let it go? Sitting there hoping he leaves her isn't accomplishing anything except making you frustrated with the situation. Just get it all out on the table. You don't have to give him an ultimatum, just be truthful and ask that he do the same. At least you'll have some answers or understand the situation better. THEN you can decide whether its worth hanging on or if it's time to let go.