Funny thing about human behavior is how predictable it can be.
He’s angry at you because that’s the only way he can justify to himself what he’s been doing. His snide and cheap comments about getting back money set aside for future events is his way at poking at you and thereby feeding his justification for why he’s angry with you.
By far the very best way to deal with things like this is to not feed his anger.
Like… I don’t know if he’s angry because you didn’t take him back, or because you told OM husband, or because he can’t be in the house with you, or because you are the reason (in his mind) that forced him to have an affair. But… it doesn’t matter…
Your response:
You can go be with OW, date OW, move in with OW, have sex with OW, spend time with OW... whatever. But not as my husband. I’m not waiting for you to decide what you can and can’t do. I’m getting out of infidelity with or without you. I am starting that journey. If you want this marriage then you need to let me know, as well as accept some conditions to make me believe this is what you want. If she’s your "soulmate" and so great – then believe me we are BOTH better off with this marriage being over. I’m fine with that, because it certainly beats what you are offering me right now.
(recognize that text? It’s the very same text as in my first post… But now you add:)
It’s inevitable that since you have decided to remain in infidelity and aren’t willing to commit to the vows you made to me and yourself that we start the process of ending our marriage. That is a formal process, and part of that process is a fair financial division of whatever debts and assets we have as a married couple. Plus we need to determine custody and how we each contribute to both this child and our unborn child’s financial safety. Plus, there will be costs related to my pregnancy. I’m not returning a single dime of any funds that might be mutual while the finances of the children is unclear. Don’t worry – this will all be accounted and will either be paid back to you OR deducted from whatever you might have to pay once the divorce process is over.
Remember – This isn’t what I expected nor even wanted, but it’s better than sharing you, or having to believe you are married to me against your will.
Maybe even follow that through with:
"Oh… BTW… Next Friday can you pick up Jr and have him until noon on Sunday. I have plans."
DO NOT partake in ANY arguments.
He demand the money – "I have told you how it will be. Please have in mind I’m pregnant and your antagonistic and abusive behavior can impact my pregnancy. I refuse to talk about this. If this is really important to you, then I can arrange as quickly as possible that you can contact an attorney that will represent me. Honestly I was hoping for more time before I had to hire an attorney, but it’s your call…"
He tells you that he had the affair because you (place whatever excuse here…)
"If we were working on the marriage we would need to address this issue. Seeing as how you are committed to your infidelity there isn’t any need to do so. Please move on."
And then you do your very best to remove yourself from any toxic situation. You don’t need to talk, don’t need to be there when he get’s stuff (have a friend be there), don’t need to visit… Lock his ability to get new food to feed his justifications.
At the same time: Let OW husband know that they are still communicating, and let his parents know that you are getting out of infidelity, and ask that since you are also dealing with the pregnancy that they maybe handle pickups and handovers of jr. and maybe try to get their son to cut the confrontation.