Here's the thing, HP....I totally get how you feel. My sitch is so very different from yours in that XH and I don't share kids. The "kids" involved in our M are his from his 1st marriage and they are all adults, so they are not keenly aware of our every interaction or even if we don't interact.
I feel much like you do. I am angry and I want him to know it and I feel this need to not be at his beck and call, though he seems to think I still should be in some respects.
However, having said that I know how you feel, I think that you are harming your sitch with S12 by being ornery and petulant. Your S picks up on all these things and even if he leans toward being on "your side", in the long run this will be damaging to him and could potentially hurt your R with him as he moves into his teen years and on to adulthood.
Please, HP, try to step back and detach and just focus on HP and S12. This is all easy for me to say, as I can't seem to detach either, even though my XH has already moved on to someone else and is living his life, but try it, HP.
My thoughts and prayers are with you.
Me 52, H53 Bomb drop 9/29/2014 Divorce from XH final 12/17/2014 Marriage #2 12/31/2019 5 adult (step)daughters (3 from XH's first marriage, 2 from current H's previous relationships) 6 grandkids