Mandy, You have a raised a wonderful son who knows the difference from right and wrong. I'm glad he told him no about dinner. Why should your son be subjected to wifey? There is no reason for him to be around her.
As for the X digging his grave, he's doing a mighty fine job of it w/o anyone's assistance.
I hope that you and your son are doing well despite all of this.
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.