((((((( silentandscared )))))))

Is there anyway that he could have heard of another child that has passed away or is very ill? My son at five went through something similar after hearing of a sick local boy, though my son's reaction was not as severe as your son's, it could very well explain it. I have to be careful that my son is not watching the news so he doesn't hear of a child dying because he becomes convinced that he will die too.
I hope this is just a stage that passes soon, I will keep you and your son in my thoughts.