Well, I am guessing that he has some deep rooted guilt issues pertaining to his children, and I have also found that many men don't want to hurt their ex's, as this person is "The Mother Of My Children".
This is not an uncommon behavior of men who do not have their priorties set.
I really don't have any advice, however, this may never become a relationship that you will be included in with his children. You may always be...the woman on the sideline.
I understand your feelings, I would feel exactly the same way.