I really feel for you because I am basically in the same position. My DS always falls in with DIL's decisions, whatever they may be, and generally they will not include us, only her own parents and siblings.
I try hard to be a good Grandmother and Mother/MIL and it really hurts, especially at Christmas. With my DIL, I just do not compare to her family- and I don't expect to, but it's so terribly hurtful when you're constantly considered second rate and not always included on special occasions.
I would say DEFINITELY do not say anything to your son. He will share whatever you say with your DIL and this could make things worse.
Your only option is to accept things for what they are. Easier said than done. I don't quite accept it, but I try to and am trying hard not to show any resentment.
I would suggest a family get together before Christmas when you can give your GC one pre-Christmas present, perhaps something funny, not the main present. Then they can take any other presents, including the main present from you, home with them for the big day.
I wish I could start a club in my area for left out MIL's and FIL's and then at Christmas we could have a jolly good time together!
Of course, the answer is also to make your own happiness on Christmas Day, doing special things with your husband and friends. Contact your Son and family by phone or Skype on Christmas Day and be cheerful and generous of spirit.
With love and a big bunch of flowers to you 