Ideas?

I'm hoping for a not-too-traumatic reason for someone with a six-ish year old son to not go to his planned family Xmas.

The family is close by, the person with the son was planning to go, but at the last minute was unable to. It's in California, so weather won't likey be a factor, and I want the person and his son to be free to travel to another Xmas celebration.

I'm wondering about Chicken Pox? Like, if the kids in the extended family all came down with it, maybe the dad wouldn't want to expose his six-year-old? But I've heard of some people so eager to get the pox over with that they deliberately expose their kids to it, so maybe the dad wouldn't stay away.

Any other ideas?