Hi marinade, my first instinct when reading about his sister is don't go there, she is probably baiting you on his behalf, are my instincts correct, no way of knowing but since she is related to him and you know about his family tread carefully.
I wish I could think of an exit-option that would give him a sense of "winning" and "revenge", without putting my child in danger.
That is the key to getting out. Think hard on how to use that strategy, a cornered Psychopath is not what you want to end up with because revenge is the tactic he will use.
Now the big question is how to do that and protect your child. Many times they get what they want like visitation etc. but don't follow through because they don't have the time or interest to be bothered. Do you think in your heart of hearts this is how he will be. Does he have any other children with other women that you know of?