When kids are this age, things that you and I take as small aggrivations are earth shattering for the kids. The fact that your ex isn't going to be there is a HUGE thing for your son. Routine is so important in kids lives, and his routine has been changed. So, if this were my son, I would see if we could figure out how to deal with this in another way. When kids have changes they react oddly - and the things they CAN control are SO much more important for them.
Imagine if your life was not controlled by you - how would you feel if plans that you were counting on were changed for you? I would think this is more the issue for your son. If your doc has found no cause for concern, then I would see if you can find a way to put the routine security back in place for him.
Maybe if your ex could come for "Christmas" on another day that might help. No way at all that Christmas can be done with his Dad? Ask hom for some ideas about what to do - "I know it's going to be hard that Daddy won't be here for Christmas, but that doesn't mean we aren't going to have Christmas and we can talk with Daddy" - something like that. You'll have to supply ideas that will have to seem like they are his. Ask leading questions - "Will it be ok if we talk to Daddy on Christmas - then it will almost be like he's here". Something like that.
As for the hands - if the has this urge to continue with this behavior, get him a little thing of lotion and after each trip to potty, tell him he needs lotion. That will help his poor little hands.
Good luck.