It is the how do I know what I know that is harder.
That
is tricky... For instance, we know that we die (science and observation) but do we really know what happens after? Nope.
Can you isolate specifically what it is that scares him? Is it the process of death or the unknown that comes after? On the other hand, if you analyze it too much with him you might just give him more to obsess about and be afraid of. Sometimes less discussion is more.
I've just tried to normalize it with my kids. Because they're older, we can talk about things like species survival ("wow. imagine if everyone who'd ever been born was still alive? there'd be no food left." etc).
Normalize, and model calmness. "It happens to everyone eventually - it must be ok." Or... "I can't prevent it from happening eventually, so I'm not going to stress myself out about it, and I'm going to trust that it will be ok." (I'm referring to healthy acceptance, and
not reckless disregard: "It doesn't matter what I do - eventually I'm going to die anyway." You know what I mean.)
I used to be really afraid of it too. Now I'm ok. (LOL I worked with a girl years ago who was looking forward to it - not out of depression, but out of curiosity! "I can't wait to see what it's like!")
