Your son is grown, married, and has a child -- whilst you still wish to dictate his social life, day by day? Did I read that correctly?
I'm sorry he pulled out of his sister's birthday, however there may be valid reasons that you've not been told. I sincerely hope you didn't complain, that way is trouble, nobody wants their decisions questioned.
Christmas: many people go away over Christmas. Posters tell of of their family trips, or those coming to stay at theirs - some other family is typically 'left behind' for a few days.
I readily admit that our daughters live within a quarter mile, we see them almost daily, however I don't believe that is at all common - people do move house after university and marriage. People have lives.
Don't cut off your nose---