I think the suggestion about discussing statistics on death rates in the US is a good one if your son is scientific/analytical. DS6 went through a period of being very worried about DH and I dying after a close friend our age, whose daughter was DS's age, died two years ago. We talked alot about different religions' views on death, how it was very unusual that our friend died at such a young age, the difference between being sick with a common ailment versus a fatal illness, and the fact that most people die of old age.
Our DS is very analytical, and it was comforting to him to understand scientifically that death is most common due to old age. I imagine if our DS went through the worry that your son is experiencing, we would probably talk about it scientifically: healthy people don't die in their sleep, and here's why.
