I was quite a worrier, too, as a little girl. My main worry was nuclear war, but there were many others. When my DS was in those phases, I would sit by his bed and rub his back and tell him about how I used to worry about the same things when I was his age and how my mother would sit by my bed and rub my back and tell me about how she used to worry when she was little and her mother would rub her back.....
I think one thing that was hard for him was feeling like he was the only one who worried, who felt the pain. Knowing that it was normal and that he wasn't crazy, and that he was not feeling the pain alone seemed to help.