Hello Liz,
It sounds like your son is having difficulties with his father. It sounds like he is pressuring your son to be his new enabler or at least Be his vat of bitterness and hate.
Being almost 18 and knowing your mom is ready to move on is probably something difficult to process as well. As in sometimes getting it and at other times not wanting to think it all the way through...
Who can your son talk to about these sorts of things? Does he have a counselor?
Good luck with everything!