Ann, sorry you are going through this and at such an emotionally difficult time as while being pregnant.
Ann, I will admit I'm a little confused. So he lives with OW. He stopped seeing his son because she forbade it, this upset you greatly. He only came back to care for son due to health issues you were experiencing. So what is the advice you are seeking? To get him to come home?
If that is what the are looking to make happen, I think you need to adjust your expectations. It is pretty clear from what you wrote what he wants. He wants to be with OW, he wants a divorce , but he also wants you sitting and pining for him so that he can come back anytime he wants.
Your best way forward is to just start moving forward without him. If he starts love bombing you again I'd suggest shutting it down with a simple: "Actions speak louder than words. You can't flirt text with me yet remain with OW. You've made your choice, stop doing this." I'm sure part of you liked the text bombing, but making yourself available for that kind of thing will not get you where you want to be.
Have you read Divorce Busting or Divorce Remedy? I'd highly suggest it if not. I'd also suggest you start focusing on yourself. Your pregnancy and staying healthy should be your focus. I'd also set your expectation that he won't be at the delivery. A man that is willing to give up his son for the OW can't be counted on to be there for the birth of this child.
You talk about not having much of a support system. Is there anyone, a sister, mom, friend anyone that could come stay with you through the birth? You mention a friend you stayed with, is this someone that can help through this time? My point with this is that I'm not sure your H can be counted on. Remember, likely future behavior can be determined by past relevant behavior. If OW won't let him see his son, I'm not sure she will support him helping through the rest of this pregnancy. So I'd start planning without him. If he ends up making himself available you can adjust accordingly.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018