There are a few that remain stuck in mlc for the rest of their lives. They are the ones that are angry, mean people and remain unhappy and unfilled for the rest of their lives. There is nothing you can do to make things better in the arena of co-parenting. Accept your xh for who he is today and move along and try to be the best parent that you can be at your end if your son should reach out to you again.
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.