You could also look into polyamory. It is possible to love more than one person, and I don't just mean physically. I mean emotionally and socially. I'm married and I've fallen for someone else, and we're trying to all work through it together the best we can. Monogamy isn't a requirement, it's more of a social/mental construct than anything officially governing life on Earth. You can be with and love as many people as you can. For some people that's one, for some people that's many. Polyamory comes with cautions though as it's a delicate line to walk, but it can be extremely rewarding for all parties involved as well.
Now, with that said. I'm concerned about your situation. I know it may seem like it's not worth jeopardizing a potential relationship that might make you happy, but you shouldn't stop living your life either. If he does want to break it off with her, he will. If he doesn't, he won't. It really is that simple, and you shouldn't feel obligated to wait patiently in the wings while he makes up his mind. The two of you aren't committed to each other, so you should go out and live life, have fun. Maybe you'll find someone else, maybe you won't, but there's no need to stagnate while you wait for him to decide.