B.A.
First of all, I'm so sorry to hear about your mother.
I'd recommend taking a gradual approach.
My parents started experiencing major health issues right about the time that one of our pets died. A friend gave us a copy of "Lifetimes" by Robert Ingpen. It was beautifully written in a way that a 5 year old could understand. "There is a beginning and an end to everything that is alive. In between is a lifetime. It is the same for people as it is for plants and animals, even for the tiniest insects." We read the book to him even though we weren't facing any imminent deaths. And then we would occasionally talk about lifetimes. When the flowers died at the end of the summer, we'd talk about it being the end of their lifetime. When we saw a baby bird that had died, we talked about it's lifetime. He gradually grew to understand that everything has a beginning, and it also has an end.
When my mother passed away unexpectedly a few months ago, we broke the news to him by explaining that just as the flowers and the baby bird had reached the end of their lifetime, so had grandma. We had also read "The Invisible String" to him, and we reminded him that even though she wasn't with us on earth, she was still connected to him.