D.B.
Hi,
First, I'm so sorry for your situation. How painful for you to go through this with your family after losing your mom to cancer, as well.
As far as your question goes, I've been on both ends of this situation myself: as a mom and as a child.
I can tell you from my own experience that it works well to be quite open about it all. If you feel comfortable, it's a good idea to start gently informing your child of the facts of the mortal body. Don't sugar-coat it but don't be negative either-- take the uplifting bits, the positives, the circle of life approach.
Slowly allow your child to digest these concepts, asking his questions whenever they pop into his mind. One of those questions will assuredly be, "Is Grandpa going to die?" And you need to be honest. Let your child talk to his grandfather as openly as the grandfather can withstand. Let this first introduction to the mortality of our bodies be as honest and intimate as it can be.
My mother did this with me, against all advice, when I was 4. As I grew up, I was the only kid I knew who didn't fear death. I felt completely at peace with it.
Now my children (ages 5 to 8) have just been to their first family funeral, and they, too, seem have a good outlook. They also just lost a 5-year-old friend to cancer (so heartbreaking) and, while they each crossed a new threshold of emotional pain, they still seem to be very balanced and healthy in their outlooks. Knowing what was happening didn't 'prepare' them for the pain -- but it laid the foundation for healing.
Placing flowers atop the coffins before they were lowered seemed particularly important to my kids -- they're still talking about it and their little conversations sound so peaceful and resolved. They do dread the pain of saying goodbye, but the closure of actually getting to say it -- and say it thoroughly -- has been integral to their grieving process. Not one of them has had nightmares or fears since the funerals, I'm happy to say.
As for me, the good effects of my early introduction to death have kept me afloat over 40 years and 19 funerals (no, that's not a typo, sad to say). Mom took a lot of flack for letting me go to Granny's funeral, but it changed my life for the better. In a way, it was Granny's last gift to me.
When mom died (cancer) she again showed me the grace within the horror and lifted me up when I might have fallen. It's the last lesson we get to pass on in this life. Done well, it can make someone else's life a less anxious journey.
One last thing: One member of our family wrote short letters ahead of time for the younger children. She knew that we (young ones) had a long life ahead of us in which we might regret missing our chance to talk to her, to ask her things, to know her. She wrote the good bits down for us. The questions we might not have until we were young mothers ourselves, she answered. It has been a lovely gift.
Hang in there. :-)