Do not pursue this man. Let him go. It is wrong (adulterous) to try to keep him.
If he is married, you should be trying to help preserve the marriage and encourage them to love one another. You should not try to lead him away from his wife, whom he vowed before God to love and cherish until death.
Even if he were divorced, do not go after him.
a) It is adultery.
b) If you were to marry him, apparently he doesn't seem to think it is wrong to see someone else while he's married. That means there is no principle in his mind that prevents him from cheating on you, if you were his wife, either.
c) He is hiding things from you.
(b) and (c) should be reasons not to trust him.
http://www.gotquestions.org/divorce-remarriage.html