If he and his baby momma want to be together, let them. I don't think you want to be the one who stands in the way of a family being whole. You need to trust that whatever happens will be for the best (and if he chooses her, you're off the hook for being the "wicked" step mom ;)). And if it happens to be true that he prefers you over his baby momma, TRUST HIM when he tells you that, because he means it. I mean, think about it: if he wanted to be with her, they wouldn't have broken up. However, if he changes his mind in the future, he's allowed to do that. You two aren't married and you deserve someone who wants only you. I know it's tough when things seem to be up in the air and out of your control, but whatever is meant to be, will be. Don't try to force something if you both are miserable and fighting over jealousy.
How long have you two been dating?
Is he certain this child is his? He should make sure of that.
Telling him how to handle the situation does come across as being jealous and controlling. I am not saying you can't voice your concerns, but telling him to tell her "it wasn't going to happen" wasn't something you should of said to him. Besides, that's not really an appropriate thing to say to a woman who just gave birth to your child......not the appropriate time to be saying something like that.
I agree with RR. If this child is indeed his then this relationship isn't going to be the best for you in the long run. You are already becoming unglued and she just had the baby.
Let him go to her. This is his former partner, and his baby.
Let him go.
Find a man who doesn't already have a partner and a baby.