Dogs can get impacted or infected anal sacs and they can even rupture. He needs to be seen by your vet. Unfortunately, it will be pretty difficult to find a vet on Christmas Eve or Christmas Day. I'd plan to take him in right after Christmas unless he seems to be in pain or starts bleeding. Then you'll have to find an emergency vet.
http://www.marvistavet.com/html/body_anal_sacs.html