Well what does he mean by this?
Is he telling you or asking you?
At that age I was spending Christmas Eve at my boyfriends house with his family.
I'd have a fold out couch to sleep on till morning - my boyfriend stayed in his room - and then after an early breakfast I'd go home to be with my family - between 7 to 8am I was back home.
That way I didn't wear out my welcome and everyone got plenty of time together.
My boyfriend didn't just invite me - his parents did - AND it was discussed parent to parent with my mom as well.
So there was plenty of supervision and no hanky panky.
There's no way I'd just inform my mom what I was doing - it had approval every which way around.
I think you and the girlfriends parents need to have a talk - the kids don't need to listen in on it - and you need to hammer out an arrangement that everyone can be ok with.
Approach it as "I know this is a family holiday and I don't want our son to be an imposition" and of course you want your son to have some home time too - let them know this.
In fact - you can reciprocate by inviting the girlfriend over to your house for New Years Eve.
After everything has been negotiated - inform the kids what the deal is going to be.