I find it a bit full on to spend Christmas with your girlfriend/boyfriend families.
To me, until you're married or in a serious relationship (ie have your own job, not relying on families for $$, living together etc), your family unit is still your parents.
But at 21yo I wouldn't have cared to spend this special day with the ILs. I would much rather be with my parents and siblings and meet up later with my BF.
Anyway that being said, it sounds like your son made a choice.
I think it is completely reasonable to let him know how you feel. Tell him you will miss them but you understand that he wants to spend some Christmas with her family. I'd also start talking about alternative Christmas.
But it's important he knows how you feel or he will always assume you don't care if he does Christmas elsewhere and won't consider alternative plans.
My DH is like that. His parents never ever say anything about their feelings or emotions so he thinks they just don't care. I had to educate DH about his parents emotions 😝
But it's on them for never telling how they feel to avoid being "that parent".
There's a world between guild tripping your kid and being open about your feelings.