No exact experience of this, but def would take this at my own pace.
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It's safe to say if he does leave for this other woman they both have a shock coming because of the rather obvious baby coming! Also he will be paying for his kids, so single life will be quite hard.
You've obviously got access to his accounts. You could befriend her, and post some pics of baby bump. She will be stalking you on the Internet anyway.
I would be practical- a warm and comfy home, are you living a happy life, do you like being a sahm / further your career if you work?
You can bide your time, ducks in rows, sort out finances, have the baby knowing you are although not taking the mumsnet moral high ground, taking things at your pace and not struggling in a bedsit if that is your option.
You can leave for a few days and let him look after his son alone for a bit. Then come back and see how you feel. Do not leave the home for good, he can move out.
You can confront him, But then you lose access to see what he is doing. You aren't supposed to know. The ow will try to make sure you do know, to hurry up events.
Nightmare though. Fucking social media.