I remember when My cat Bear was in about his 18th year. I worried about him daily. I even woke him up from a sound sleep every time I got home from work just to make sure he was still alive. (poor thing probably thought I was nuts). I had no basis for this except his age and my imagination. He lived to be almost 20 so he endured this paranoia for 2 full years.
It could happen at any time to any person OR pet, so always try to live for today. Make this his best Christmas and then make next year his best Christmas too.