I found the book Badger's Parting Gifts a good way to help a child understand death.

I'm so sorry for your loss. I lost my Dad 6 1/2 years ago and it is still hard on everyone. Your son understands more then you think. He is grieving just like any other person. Our kiddos process things differently, so he is still processing the loss of his Grandmother. I would try putting together a social story about what happened to explain it to him a little better. How Grandma got sick etc.if you are religious, them tell him about Heaven or whatever you believe in so that he knows she is gone but someplace safe etc. I wouldn't make scary or sad, but I would not take him to her grave. It would really be too much for him. I really believe he is grieving.