If you find yourself in this precarious position, you have a lot to think about.
Before you can decide what to do, you need to take a good, long look at yourself and figure out how you got yourself into this situation. Remember to be honest with yourself- there is no point in lying to yourself, somewhere down inside you know the truth, anyway.
Perhaps this is about you? Have you suffered from low self esteem and been taken in by the attention he gave you? Do get an ego boost feeling that you can take him away from someone? Do you frequently find yourself in impossible relationships, perhaps indicating a fear of commitment or true one-on-one intimacy? If that is the case, then this relationship is not going to help you move on.
Just as important as your motives are his motives. Perhaps he does love you. Perhaps he is a good person who was unhappy and found himself in a situation where emotions got the best of him when he met his true love. If that is the case, expect him to do something about it. Give him a time limit, and stick to it.
If he claims an unhappy marriage, it's just not enough of an excuse. If he is so unhappy that he felt like looking for another lover, why didn't he just settle things with his wife first? Don't allow him to treat you like a consolation prize, or a side hobby to help him get through his rough days. If he says there are complicating issues, such as children, family, financial problems, etc, then is he really in a position to become involved with someone else?
If he isn't making any real efforts towards resolving his marriage to be with you, then he is not giving the respect you deserve and obviously not able or willing to fulfill all of your needs. If that's the case, no matter how much it might hurt, the reality is you'd be better off finding someone who is willing and able.
Another thing to consider is, has he done this before? If having affairs is a hobby for him, no matter how you feel about him, you need to consider if you really want to get mixed up with him. After all, even if he leaves her and comes to you, are you sure he won't continue with his extra-curricular activities behind your back? Do you really feel you could trust him? If not, does your relationship have much hope, even after his wife is out of the picture.
Don't ever let feelings overrule your self respect and common sense. Thankfully, love is not as rare a thing as the movies and romance novels make it out to be. As humans, it is not that hard at all to love someone in our lives when we have a connection. The hard part is finding relationships where mutual respect, dignity, courtesy, honesty, cooperation, trust and courtesy are companions to love. Without these, love is not enough. If you both feel you have these, then expect him to do something about it, soon. If you don't feel you have these, or if he hesitates to give them to you, then find the courage and strength within yourself to end the relationship and find them elsewhere. After all, you deserve them.