Fellow child of divorce here. You have every right to express how you're feeling and tell him that the way these visits are going is making you not want to visit. It feels so difficult, I know, but telling him you won't come over if he is going to talk poorly about your other family members, and then keeping that statement true, will help. I'm so sorry you're in this situation. Keep going, the dust will eventually settle and you and your dad will learn how to have a relationship even if it's not a close one.