First of all, it's your choice whether you get back with him or not. You obviously don't trust him not to leave you again, and you don't trust him to leave his girlfriend now. You don't even know if he's lying to you right now or not.
WHY would you want to be with someone like this? It will only bring you heartache and pain. You're worth so much more than this, and you're selling yourself short if you let him treat you this way. You deserve to be treated like a queen.
Relationships need loyalty and trust. He isn't loyal - he's proven that already - and you don't trust him. What kind of relationship is that? Just because you're pregnant doesn't mean you don't deserve the world from someone, and it's obvious he isn't able, or willing, to give that to you.
If he wants to be involved in his child's life, I think that's great. But I think you should wait on getting back with him until he PROVES that he's worth getting back with. Don't allow yourself to be used by this guy. You're going to be a mother - probably a single mother - and you need to learn to stand up and be strong.
It's better to be alone than with someone who treats you so terribly.