my little boy went through this and i didn't know how to help him. After assurances from most doctors that it was just a phase, i finally got some help from a brilliant (sadly now retired) health visitor.
She came round and asked him to put all his worries on a piece of paper. He wanted me to leave the room while he did so.
She told me that as he scribbled - with mainly black crayon - he was saying things like: and that's the fire, and that's the volcano; and there's the plane that will crash on our house etc.
Then she asked him what he would like to do with the picture, burn it, bury it?
He decided to fold it up very small, stuff into a old film canister and then he wrapped yards of black tape around and around (so they couldn't get out!). Then he put it in the bottom drawer of my freezer!
It is still there, and as he's now 15 we can laugh about it, but over the following months, he would come and check it were still there and shake the box, saying "Ha! You can't get ne now worries!"
I don't know if we were just lucky but it definitely worked for him. He was back to his old self almost immediately.
Incidently, I found out later on that a school friend's grandad had died and though he wasn't really close to this little lad, it had started him thinking about death and dying.
Good luck x
P.S. Hi lil75 - I have been wondering how your little boy is doing. x