Ah,
Some of us forgot.... once our kid hits 18, they are legally adults and suddenly expected to be emotionally mature, making all the right decisions and no, we shouldnt challenge thesse decisions snd any potential impact on family init.. because he's 18, duh.
Noting not many suggesting the girl be encouraged to spend more time at his place and a number of comments imply its the posters problem from being one of those mothers to eon suffering within a busy noisy family and yes thats family life for you, the good, bad znd ugly where you learn to negotiate, compromise and learn about love in relationships.
He's 17 , in the throes of love and lust.Maybe the girlfriend is shy and prefers it at hers,or maybe he's worried you might not approve. Finally it might be less supervised, ie they can go to her room all day without interruption by nosy and noisy siblings, but that's family life...
If there isn't anything other the the hullabaloo of family life going on at posters, our poster is right to lay down the law and say Christmas day is for family and he needs to be there for his.
He can pop over to gf on boxing day, but suggest they come back home for dinner/supper and just meet your lovely noisy loving family.
I'd also start gently quizzing him about what its like at girlfriends house, making no judgement comments and listen to what he's saying, it will come out with time but stay positive... he'll be back