Your going to have to let your son go. He was old enough at 18 to know right from wrong when your ex made his choices.
Now at 22 your son choose to live in a dysfunctional environment with his father let him.
But let him know that your door is always open, but move on for your own sanity you can't keep worrying about him and the choices he made so far.
He moved in with you and saw what you could provide for him a calm home filled with love and for some reason he wanted to go live in a war zone with two damaged people.
As for your ex at work ignore him when possible as in don't even acknowledge his even in passing, he is now the stranger you work with.
