A.W.
N.,
I guess I would ask if you are worried about your little boy, or are you worried about you being seperated from your little boy? I had to fly to Boston for a week and leave all four of mine with my mom. It was worse for me than it was for any of them. When you were little, did you get to stay with your Gramma? I did, and it was always great, even if Gramma was exhausted when I went home.
Make sure he gets to take something that he wants to take, a toy, a movie, a pillow or blankie. I always included medicine, just in case. Most Grammas don't keep kid's meds handy, and since you have it, she won't have to go buy it. There are some things to help your peace of mind, too. I always had a medical release form for my mom, JUST IN CASE, one of them would wake up with pink eye or something. She could take them without waiting for me.
Relax. I know it is hard. It is still hard for me to leave mine for more than a night, but it will be a great break for you, and a nice treat for both your Mom and your son.