As many have said, I would ask him if he has any questions about what happened. Fortunately you can explain that sometimes when we get old our body just doesn't work anymore. Also, with the faith that you have you can lean heavily on that and explain that you do not have all the answers and sometimes we just have to believe what we believe (ie have faith).
My son had just turned 5 when my Grandmother passed. He saw her often and understood that she was sick in the hospital, and visited often. Don't be surprised if he doesn't have a lot of questions, and doesn't seem too affected by it. I think kids have an easier way to deal with things because they don't have all the thoughts we have as adults. We told a lot of stories about Gram Gram and let him attend the wake and touch her. We also have a picture of her that he refers to when he talks about her.
Good luck, & God Bless you and your family at this difficult time,
J.
SAHM of 1 1/2 year old and 6 year old boys