Your boyfriend and his mother needs to cut the apron strings....I have had a similiar problem so I understand! Even though you are not married, getting a place of your own implies a certain amount of committment and therefore you should share your holidays together (wherever you choose). Before we all grew up, we went w/ our parents and did what they had us do. Now that we are adults, our parents have to expect a certain adjustment. You can't possibly do what both of you have always done let alone add your own traditions. The merging of two families (whether official or not) requires compromise and adjustments.
Both my family and my inlaws always celebrated on Christmas Eve so that is a crazy hectic day for us. My inlaws used to have dinner at the grandparents (with the WHOLE family), then did Christmas morning at home, and then went back to the grandparents for the rest of the day. After grandmom passed away, the girls (my MIL and her sisters) took turns hosting dinner for everyone, then it moved to another days just before Christmas but my MIL still wanted us over for Christmas Eve and Christmas morning too....and the day after because it is my FIL's birthday. Doing so would mean nothing but non-stop running for us.
I already had a son when I started dating my now husband and I wanted to keep my son home on Christmas Day so if his dad didn't get him, he got to stay home all day and enjoy the holiday (something I never got to do as a child because I had to leave on Christmas Day for a whole week, every year and hated it!).
I told my husband that I would run every other holiday but Christmas Day is mine to enjoy at home...that is what we do. We attend (sometimes host) the big family get together, we visit both sides of the family on Christmas Eve (sometimes we arrange for some of them to come to us) and then we stay home on Christmas Day. Anyone who didn't get enough of us the night before is more than welcome to come visit.
Since your boyfriends mom gets Christmas Day EVERY year she should be more than willing to budge on Christmas Eve. I personally think you should look ahead to when you have children and either start implementing what you would want then or at least reach an understanding that you will do "xyz" now but once you have kids you want to do "abc". Otherwise it will become.."well you have always done this why are you changing it now". I also think you boyfriends desire to separate for the holidays show a lack of commitment and you should probaly discuss that with him.