This is getting ridiculous now. Let's break it down:
He spent first 4 christmases with his parents. Last 2 with me.
Whether this was with her family or not is important, but not clarified.
This year his DM insists he spends Christmas at the family home and he thinks it’s the right thing to do.
I don't think it's unreasonable to want to see your own family on Christmas Day when you've missed the last two. What is not clear is whether the OP has been invited as well. Perhaps she was, and refused in order to be with her own family and sees his decision to go without her as him not wanting to be with her at Christmas.
Until she clarifies, we can't know who is being U here.
I suggested, Christmas Day with me, then Boxing Day and 27th at his parents.
Christmas Day just the two of them? Or with the OP and her family? And did she intend to go with him to his parents on Boxing Day and the 27th, or not? Again it's not really clear about exactly what is on offer on either side.

He and his DM have suggested Christmas Day and Boxing Day at his parents is more appropriate since we are not married and child free.
It sounds as though he and his parents are just putting their foot down about having their 'turn' on Christmas Day. In the name of fairness I can't argue with that. But if the OP is not invited then that is shit.