Firstly, I have been there, my DS was only 3 on the first christmas after we seperated, and it isn't easy but it will be fine :-) and you'll find a way through.
My DS and I stayed with my parents over xmas so I had plenty to keep us distracted - is this something you could do, with friends or family?
I admire the way you still have enough of a friendship with your ex to contemplate inviting him over, my exH and I had a messy divorce.
DS is now 7 and this is the second time that he has been with my exH on Xmas Eve (6pm) - Xmas Day (4pm).
The first time DS was 4 and we did Christmas Day on Xmas Eve in our house which in hindsight was selfish on my part as my DS could have got very confused but luckily he was still young enough to not notice.
The last two years he's been with me and my DP until 4pm Xmas Day til 6pm Boxing Day - my ExH decision to fit in with his new family's routine etc.
I'm still deciding what to do this year as Christmas is very important to me and my DS has openly said to us that he wants to be at home, not at his Dad's but I know it will be a nightmare trying to explain this to his Dad without him flying off the handle. One to deal with in two years me thinks.
Anyway, I agree with the other poster that you should think carefully about how your decisions for this year could affect next year when both of your circumstances may be very different i.e. you could have another partner and possibly other children in the mix which would make sharing time at christmas together difficult.
I have explained to my DS that Santa knows that he won't be at his normal home on Christmas Day so he makes another special delivery on Christmas night for all those children with two sets of parents and that we will have christmas a day later this year.
I try to think of it that my DS has two Christmas Day's which must be every child's dream!
We do lots of special things on Xmas Eve and I don't want to not do them this year so we will just postpone them for 24 hours.
I get myself in such a tiz about it all but I have to remind myself that it is just a date on the calendar and it doesn't matter when you decide to do those special things, just that you do them.
Your DS will be happy no matter what. And there's always mulled wine to carry you though lol!