It is hard when one parent has the child for an extended period of time that covers the days of the holiday. Just tell him to write a letter to Santa and tell him that he won't be home on Christmas Eve and could he just leave his gifts anyway.
You might want to make a story up if Santa also finds him at his dad's too. Maybe Santa didn't want him to not have some of his stuff to open. If the ex forgets Santa though that might just be awkward.
That way he'll stop stressing over Santa finding him. Plus the ex might mess it up anyway.
My daughter and her ex had a written agreement from Sunday at 6pm until the next Sunday at 6 pm. They swapped the child at the police station. That way we knew he would show up. He was really afraid of them and he knew if he didn't keep strictly to the agreement they would come looking for him to arrest him and take the baby to my daughter. It didn't matter what days happened while he was at one or the others, no exceptions were made.
I just think he needs to focus on his dad and having fun and not be stressing over not seeing you. Millions of parents out there are not going to have their kids on Christmas Eve and some on Christmas Day too.
My friend's brother ended up divorced and his ex moved to another state. He gets his child from the day school is out until the day before it starts back up again. The whole holiday and he is't going to drive 18 hours to let her have Christmas with her mom. They just have the celebration a different time too.
I hope you can settle your mind about this and that your son can too, it sounds like he is really worried about you and loves you very much.
When he comes home have Santa stuff ready, a nice dinner for both of you, Christmas music playing and after eating watch some Christmas movies.
One of our Christmas traditions is sitting and watching "Blizzard". Whoopi Goldberg is the voice of Blizzard, the offspring of 2 of Santa's main reindeer. It is a heartwarming movie about finding true friends and overcoming obstacles. Maybe you could go caroling earlier in the week to some of the nursing homes with a group of friends. Anything that cements the reason for the season and brings you closer together.