sassy - I see so much of my own situation in what you describe. It's a power struggle, really. Your mother is trying to make a statement of some kind to your son (and to you). It's complex and I won't even attempt to analyze it.
I'd like to offer another perspective, though. I really do understand your frustration, my mother has made me crazy doing this type of thing to me. but I've realized that my daughters are watching everything I do when it comes to my relationship with her, and I have a feeling that one day it will come back to haunt me. I've been thinking a lot about this lately, and I've realized that it makes my daughters happy when I'm kind and generous to their grandmother, even when she does awful religious-related things. they respect me for being the bigger person. As they often say, "mom, she is still our grandmother and we love her".
Try and see the situation through your son's eyes. It may be very difficult, but do what you think he would like you to do. In the end it will make both of you much happier. Don't let your mother ruin what should be a wonderful time. Let her talk to your son if she wishes (and he agrees) - fighting it and getting upset will only hurt you...and in the end it will affect your visit negatively.
You will be doing a lot of fence-mending and rebuilding during this visit. Let your son see you in as positive a light as he can.
good luck. *hugs*