Cherry, it sounds like you are handling things with your usual strength and grace. Good for you. Your H clearly has some thinking to do here. And I think you are right to focus on you, your son, and remain lovingly detached.
I also think, whenever there is an opportunity, restate your clear boundaries. You don't want to go overboard on these, but IMHO, he just needs to know that Cherry won't tolerate an open marriage and has some thinking to do herself about all of this.
It will be interesting to see how he is in coming weeks. If there have been particular behaviours you associate with him being in a PA, do these change? Was he distant with baby and becomes closer? Is he volunteering information to you? Is he in more, and more inclined to chat? Does he share his whereabouts with you?...and so on.
Bear in mind that it is pretty unusual for A's to be cut off and to end completely. Their addictiveness tends to draw people back in. Three times D - that's a lot. Is she much older than your H. If she's three times D and in her twenties, that's an awful lot. I don't think I've even seen stats on fourth M's...
You're doing so well Cherry. Keep looking after yourself and baby my friend xx
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus