5yo DS worried about death/dying

[Cheeseismyweakness]

Help!
Any tips for how to manage my 5yo DS who is currently very worried/ preoccupied about dying. He is a bit of a thinker and in the last few weeks has stopped going off to sleep so well at night, often coming back out onto landing in floods of tears cos he's worried about death. He keeps asking me when we are going to die, why we have to die and saying that he doesn't want me to die.
Also questions like where is God/heaven etc, which are slightly awkward given I don't even know what my own beliefs are..
Wonder if he's seen a little too much on tv like Star Wars recently (where Darth Vader takes his mask off at the end)- too young for this I am thinking with hindsight...
Any advice gratefully received as it breaks my heart to see him so worried and upset..

My DS went through a similar phase after his Great Granny died, plus a very morbid interest in funeral parlours and graveyards. I always hark back to a book I had as a child that talked about bodies getting worn out and stopping working. Maybe you can reassure him that it's usually old people who die and he is young/strong/ healthy. DS was also reassured to be told that we'd appointed guardians to look after him in the very unlikely event that something happened to both of us.
Can the God/ heaven stuff be phrased as "some people believe" if you're unsure about presenting it as fact?

Just wondered if anyone else had any advice about this? My 4yo DS has just been told about death/dying by my FIL (thanks for that) and is also now worried - I don't believe in god/heaven but he's also being told that dead relatives are stars in the sky etc!!

I wondered if anyone had any good books to recommend, or some good phrases/terminology I can use to help him understand? Everything I think about saying still seems too advanced for a 4yo.