I'm sad this has happened to your son - and to you OP. I'm sorry but it seems like your partner IS a bully - it's not the first time he's acted towards your children like this and both times he has behaved like a bully, that you've mentioned, his own children have been there. What is he trying to prove to either his children or yours?
The sad thing is he doesn't seem to care about the consequences of his actions.
- Instead of hurrying to the pool to pull your son out again, and profusely apologising, he stands back and lets his pregnant partner do so;
- He seems to care more about how you "humiliated" him in front of his children than the fact your own child was in trouble. You said of your son: I heard his chest go funny right away and he needed his inhaler and
my son he could barely breath after took me a good twenty minutes to get his breathing stable.
What was your partner doing in those 20 minutes?
- He treats your children differently (less kindly) than he treats his own children.
- He's blaming YOU for his own poor judgment and lack of emotional intelligence.
- I'm glad he's spoken with your son now, but that is the very least he should do - and he's still behaving badly to you as he's not accepting responsibility for his actions.
You are in a tough spot as you are soon go on maternity leave and the timing is reminiscent of what happened with your children's partner but you really can't let this go, I'm afraid.
At the very least I think you should ask him to leave for a few days and give all of you, but especially your son, some breathing space away from him. You will all be in eggshells anyway.
I'm sure you will be careful when you speak with your son but please reassure him that he is not to blame in any way for what happened before, or what happens between you and your partner from this point.
You say you have some family support? Are they located close by and able to meet up with you/your children during a socially distanced walk/picnic etc and generally keep an eye on you all, and especially support you?
BTW you sound like a strong woman. You got through bad times before and made a good life for yourself and your family. With some family support maybe? you can do it again.
The best thing is your children will know you are 100% on their side.
Best wishes and good luck with all this OP. You have my sympathy. 🌹