He dated someone else a month after we broke up even though I was pregnant with his son :-( am I right?

It wouldn't matter if he'd broken up with you, or what the reason was. The point is, you two were broken up. That means both of you - yes, both of you; pregnancy or not - were free to date whoever you wanted. If you want this second try to work, then you need to let it go and move forward. And if you still feel like you can't trust him, then baby or not, why are you with him?