Just reaching out to say I totally, totally get it. In fact, I'm still LIVING with my husband because I would rather slog through the difficulties of trying to make it work with him than lose out on any moments, Christmas morning being a biggie, with my children.
As Unchien says-- from a pure negotiation standpoint, what do you know about him that he would value that you would not, so much, that you could trade? And/or, do you think he's playing you this way, somewhat-- he knows how important this is to you, so is pushing here in order to get something else he wants?
Another thought... my H and I both live far away from our parents (who also live far away from each other). We always alternated between flying to my parents' and flying to his, and trying to coordinate schedules with our siblings and their respective in-laws too. One thing we always did was have Santa come Christmas morning no matter where we were, even if usually we'd open our own family presents to each other before we left so that we wouldn't have to pack all the presents on the plane.
One interesting thing is that my H's brother will normally have Santa come on the 23rd or 24th, before they leave their city to drive to his parents so that they don't have to pack Santa's gifts in the car. His kids didn't bat an eye. So, there is no reason that Santa can't come to your house on Christmas Eve morning so that you still get that special time with him. Blow it all up just like it is Christmas morning. S2 will get TWO Santa visits! (And I'm going to guarantee that the one at your place is going to be much more magical. I can't imagine knowing what I know of your ex that he's going to work too hard on finding the perfect stocking stuffers.)
The other thing I wanted to note is that as my kids have gotten older, the part where we open our own family presents to and from each other has become more and more special (just like I remember it myself growing up-- we always opened family gifts on Christmas Eve). They are just as excited, if not more so, to have people open the gifts they chose than to open their own presents. When they were younger I always thought that Christmas magic was wrapped up in the magic of Santa. And of course there is a big piece there. But there are so many elements-- decorating the tree, making gingerbread houses and cookies, vacation days, hot cocoa in special Christmas mugs, choosing and wrapping presents-- that I bet you can break down what elements are most meaningful for you and what family traditions you want to create with your son-- and you can make that magic for the season, even if you end up having to miss that one night.
(And, again. I GET IT. It makes me want to be physically violent right now that my H even made me need to THINK about this. I would just try to tease apart why this is so meaningful to you, what elements are the most important, and then figure out how to make those all happen for S2 and you whether on Christmas Eve or Christmas Day or a week before or after.)
((Scout))
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing