Mandy, I am sorry that your son's father has not kept in touch or supported him. Some day, most likely on his death bed, he will regret the way things have gone down. Your son is a very wise young man and you've been there for him through thick and thin. Your son will always remember that.
As for your "X", he will remain married to the skank because it is easier to do so rather than live on his own and actually grow up. The skank is most likely a mother figure to him and that's what he needs because he is a child in many ways.
As for her comments on the network, what a piece of work. If that doesn't take the cake, nothing will.
Mandy, you are doing a great job in being there for your son. I do hope that you and your son are doing well. Please stay in touch.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.