Greetings Toledest!
Although I have been DF'd for 4 years now, recently my own parents have taken a firmer stand and in their words "chosen to limit their contact with me" (their letter is really quite interesting and I will make it available on my website some time) due to my letting them know more firmly my views and opinions about the Organization. This is unfortunate for them and for my 2 year old daughter and my wife (non-JW).
I say the above to say that I can emphathize with you.
When I read your post, my first thought was that, well your son's grandma doesn't celebrate Christmas anyway so even if he could go and visit her, he wouldn't have anything to tell the other kids or probably he would be embarrassed to tell them that.
But I suppose what you are upset about is the lack of "normality" not so much for yourself but for your child. Unfortunately, there is nothing you can do about that. The fact that your mom is a JW means that there will not be a "normal" grandparent-grandchild relationship as "normality" is what we assume it to mean. (Although what is really normal after all? There are a LOT of screwed up grandparent-grandchild relationships that are not Jehovah's Witnesses.)
From a practical approach, the best thing that you can do is to be the best that you can be and make things the best that you can. So your grandson may not be able to say "yeah Christmas at grandma's was awesome" but hopefully he can say instead "Nah, my Grandma doesn't celebrate Christmas but WE DO and we had an awesome time, let me tell you what me and my mom did....."
Now, in my opinion, a 13 year old kid should be sophisticated enough to both understand the situation here and more importantly to be able to have a conversation with his grandmother on behalf of himself.
He should be able to say to his grandma something like this:
"Grandma, I know you are a JW and I respect that you have your own beliefs, even if I don't agree with them. I hope that you can respect me in the same way. I don't agree with or understand and I will never accept your shunning Mom. I can only hope that New Light, or circumstance or Love itself will one day make you see how wrong you have been and how much you have lost all these years, missing out on your time with Mom and with me. But having said that, your own religion teaches that you should NOT shun me simply because mom is disfellowshipped.
I would like to have a relationship with you. I would like to spend time with you and even visit with you from time to time. I am old enough now that I have my own opinions and beliefs. If I visit I would like not to be made to go to the Kingdom Hall or to be preached to and I think I am old enought to have that choice. If you can accept that, respect my wishes, then I would love to have relationship. What do you say?"
I think a 13 year old should be able to express his views and should have them in the first place.
-Eduardo