with DSS we do alternaate years.
this year its our year to have him chrissy day.
our situation is slightly different as his mum lives 7 hours away.

but the year we dont have him we put off the whole chrissy thing till australia day and have a huge chrissy lunch and pressies on that day so he doesnt miss out on anything.

in your situation id be telling xp this is what we are doing you are welcome to come and enjoy the day with us, or you can have dd for boxing day or something.
or arrange a few hours to drop in and see him.