OP, can I comment on the recovery from this affair on its own please, without reference to his other son?
So you are about 18 months-2years down the line from discovery? I am interested in how you're feeling about the affair, the OW and your H.
You don't say anything in your posts about the journey he has made to analyse his own fidelity. From the way he is behaving, it would suggest that he has never truly taken responsibility for his infidelity, especially if after all this time, he is pursuing the "she chased me line". I think your journey also got stuck at some point if you're echoing that irresponsible idea too.
I am 2 years down the line myself, but it sounds like you, me and our Hs have travelled very different paths to understanding.
In the early days after my discovery, unlike you, I wanted to verify everything my H was telling me. He was an open book with his phone bills and showed me various texts and E mail correspondence between him and OW. It was these irrefutable bits of "evidence", together with his recollection of conversations between them, that allowed me to construct the story of the affair. Had the OW offered to send me E mails, I would have jumped at the chance. It was terribly important to me to verify what I was being told, rather than believing an understandably subjective account from a man who was abjectly sorry and distressed, but also minimising his own culpability.
It soon became clear that all this hand-wringing and saying sorry on his part was getting in the way of cold hard analysis and understanding. So my H went off to counselling on his own and read voraciously about how it had been possible for him to discard his values and risk the relationship that meant everything to him.
All these things had the effect of him taking complete and total responsibility for his actions. He had never once blamed me or our marriage and has never wavered from insisting that ours was a happy marriage, so the problem was him and not me or our marriage. In the early days however, he did have a tendency to blame the OW for things he did - and that had to stop.
In the process, he confronted things about his character that had led to this - selfishness, complacency, dislike of confrontation and difficult conversations - and resolved to change them. Consequently, those flaws have been banished and he is a much better person for it.
As for me, I would have accepted nothing less.
You see for me, I couldn't have stayed with a man who wouldn't take responsibility for his actions. I couldn't have respected him and since my respect for him had gone after the affair, only seeing him confront things he found difficult, restored that.
I'm curious how you can love and respect a man who is so obsessed with his own pain two years on, that he is neglecting yours and the OW's. He is still blaming and appears to be doing nothing about his own character flaws. Wearing a sackcloth and ashes is actually a controlling behaviour that stops the difficult stuff from being confronted. I'm surprised that you are not seeing this and that you're allowing him to control you in this way.
It is understandable to hate the OW and not forgive. To pick up on earlier Christian references, if she has never once said how sorry she was for betraying your friendship, it is pointless and impossible trying to forgive her. She was wrong to have an affair with your H and wrong to hurt you.
However, it is always erroneous to blame her more than your H. He could and should have said no to an affair.
What I learned from all this too is that you cannot forgive until you know all there is to forgive. You appear to have launched on a path to forgiveness without even wanting all the information. I think that's a huge mistake and will come back to bite you in the end.
I assume you want to affair-proof your future marriage and ensure this never happens again? What I can also tell you is that this never happens until the unfaithful party does some really necessary self-analysis and resolves to change their character. And it never happens if the betrayed party puts their head in the sand and hopes for the best.
You ask what can you do? As regards the OW and his son, you tell him that you will not be dealing with this any longer. He must take this on. If he doesn't - well it's again a matter for you whether you'd stay with someone who ignored a child, but I couldn't. I couldn't respect someone who did this and therefore wouldn't want a relationship with someone I didn't respect.
And unless you're prepared for this to happen again, or stay with someone who isn't worthy of respect, refuse to collude with his complete abdication of responsibility for the affair.
Sadly, you have made yourself more vulnerable by having another baby before he has done all the necessary work and perhaps because of this and your faith, he doesn't take you seriously and has no incentive to grow up and face this; he knows you'll stay with him whether he grows up or not. That is a lost opportunity for you and a real shame, but I do hope you start your own journey at some point and build enough esteem to start asserting yourself.