Have you read Divorce Remedy?
Stop asking him about the PA for now. If he's denied it there is NOTHING you can say or do to get him to admit it. Just assume it's happened and now be the best woman/mother/wife you can be.
I think now would be a good time to concentrate on you and the things you like and remembering who you really are and not this freaked out scared woman. Be confident, like you own the world. I just recently started this and I mean very recently with more consistency--good results. It's very sexy.
Concentrate on your baby boy, sounds like you went through a lot and now that he's here, enjoy every second, no matte what is happening around you.
My H had an EA and a PA too. I'm not sure how PA it was, but the EA is bad enough. Still working on recovering from that but at this point he says it's over and I'm delusional.
My goal right now is no fighting for three solid weeks, no fighting of any kind. And that goal includes asking/talking about OW at absolutely NO time at all.
Let that go for now. Table it. Come back to it when you're in a better place.