If he really wanted to get back together, and was actually sorry for cheating on you, he would not be sleeping around.
He's a total shit, and try to guilt you by being legalistic and saying you're 'on a break' so he can do what he likes. He's showing no empathy for what he's put you through, never mind love.
But he is, thankfully, showing you exactly who he is - a repeat cheater, who doesn't want to support you.
You are so much better off without him, if you were back in a relationship with him he would be sleeping around, just less blatantly.
My relationship ended while I was pregnant, and it was very scary going it alone, but once my DS arrived I realised it was all so doable alone, and the reduction in stress by not having to manage a relationship with his father was enormous.
So sorry that he's so awful, so sorry that so many PPs are saying you're unreasonable to feel hurt/betrayed by his behaviour, but you know who he is now.
And congrats on your soon to be new arrival! I hope you have a friend or family member to be your birth partner - do not let this odious creep near you when you're vulnerable!