If this were me, I'd pay whatever I had to to take care of this myself. Like if there were taxes XH owed and I was getting the brush-off, or if it was making my life more contentious or bothersome to try to get him to pay what he owed, I'd rather just pay what he owed myself in order to leave him out of it. Even if I needed to borrow the money from a family member. I'd just not deal with him. I've done this a few times before. It was easier to just go without something for a few months to find the money to pay for something than to try to deal with XH.
Technically I am doing this year. I expect to owe a lot on my income tax because when I took over the mortgage, it was only in April, and despite paying out most of my mortgage to interest every month, I still didn't hit the magic number to itemize. So I'm looking at owing around 1200.00 that I don't really have to put out.
XH and I were still on our mortgage together for Jan. to April last year. He did not pay one cent of the mortgage though we were not divorced yet. I took over all of it then. The only way I can "claim" that interest is if he and I file a partial year joint return. THEN, I would hit the number to itemize and not owe nearly so much. But the thing is, I would rather pay the money out all by myself, even though it's not "fair", to avoid dealing with the anxiety of having to deal with him.
My sanity is more important than money. I don't care anymore about the "principle" of right vs. wrong; I only care about what gives me the most peace. So even if I'm paying what he should pay, I don't care. I find the money and pay it and move on.
I don't know how much you're talking about paying out here, but have you considered just covering it yourself in order to detach even more?
As for the relationship with the son, I hear you when you say that you really think that your son would benefit from seeing his parents get along. Well you can't force someone to get along with you and repeated attempts to try to connect are only probably making him angrier or more set in his ways. If you let it go you are letting your son know that you are accepting of the outcome. You don't have to like the outcome to accept it. But if every thing you do keeps blowing up in your face, you have to stop lighting the fuse, so to speak, and just let everything up to the universe or god or whatever you have faith in that it will all work out :-)
M45 Bomb 6/09; EA 6/10; Divorced 1/11 Proud single mom of 7 little feline girls and one little feline boy "Fall down 53 times. Get up 54." -- Zen saying