I went through a similar situation with my ex boyfriend. We ended up back together but I broke up with him two years later because the new girlfriend he met while we was separated came back into the picture and he began cheating. But If u love him, take it slow but I would see where things could go. You never no, he may fall in love with the new baby and become daddy.! Good luck!
If he hasnt been there for you*
Ohh I'm sorry... that must really hurt that he is dating while you are bearing his child. If he doesn't value and appreciate you then I would definitely start letting go of that relationship and move on because someone is going to love you and your baby much more.... you deserve it. If he isn't there for you now when you are so vulnerable and going thru such a life changing experience that is also so exciting and amazing, then he doesn't deserve you or the baby in his life.