Let him get a babysitter. he needs to tell you who it is and give you the phone number and vice versa.
I know you feel like you are doing the same thing by not choosing to have time with S, like he did. But you know what? You have like everyday with son! he has minimal time. So don't you feel like you are on equal footing when you chose not to be with your S on new years which is not a kids holiday.
Now tell him you made plans and you are sticking to them. He's going to have to handle it. let him find a sitter. Your S will be fine. And he probably won't be able to find one or have ot pay out of the butt on that holiday and he'll have ot make the saacrifice.
