money train
If the car is your in your name be upfront with him- I can't afford the car or his insurance anymore. If he offers to and actually makes payments for now go with that. If he can't afford it or all of it perhaps sell him the car for the remaining payments and transfer the car in name to him.
moved out?
If your relationship is basically over I would sever financial ties as soon as possible so there's no excuse for him to be in the picture.