Whatever you agree to, try to make it so that you’re happy with both ends of the agreement, as next year you could be in his position.
FWIW I know it’s hard the first year, but if you can let go of the specific day being so important and arrange things so that you both get some special time with them it will be fairer.
Depending on their ages, is it the Santa/stockings/excited morning part that’s most important? Or spending time with extended family etc? Boxing Day can be done on any date, it’s just another festive get together, so as long as your family are understanding there’s no reason you can’t have a traditional Boxing Day family get together on 28th or even Xmas eve.
Many people split the day eg kids with you Xmas eve and Xmas day lunch, then go to dads later and for Boxing day. Then reverse next year.
It’s not as bad as you think, especially if you have other family around.
We have exH here on Xmas morning and for lunch, then the kids choose if they want to stay with me later or go home with him. I’m usually knackered by then so happy to have some peace!
And the year they stayed at his Xmas eve and he did stockings was brilliant - I didn’t have to stay up late until they’d gone to sleep or wake at the crack of dawn. Bliss!
It’s hard adjusting to new routines but just try to remember it’s just a day. Christmas is a season and you can celebrate for a week or more, regardless of the number on the calendar.