It is never a good idea to date someone who is married. They are legally bound to another person. Many married individuals in this predicament say they’re going to leave their spouse but they often don’t. Often they have no intention of leaving and are simply lying.
If you’re going to remain in this relationship, it’s important to have the right expectations. Expect that he will not leave his wife. In all likelihood, this relationship will be short-lived and not serious. His statement of leaving his wife could be taken seriously if he were finalizing a divorce, but this is not the case.
You stated that you want this “man in your life forever.” Know that if you move forward in this relationship (assuming he leaves his wife and family) that you will also have his family in your life forever. He will be forever connected to his current wife because of their children. Are you okay with sharing the rest of your life with his current family?
Dating someone who is married is hazardous and painful, not always but usually. You obviously have strong feelings about this relationship but try not to let this infatuation blind you to the reality of the situation. Please take care.