He could just be a worry wort. By brother was the same growing up. He hated christmas night for years and wouldn't sleep, Mum always assumed it was because he was excited, but he usually stayed up worrying because a strange man was coming into the house! lol He was worried about santa because he didn't know him!
He is 20 now and not such a worry wort, but your son could just be someone that thinks a lot more then others, he over thinks things. I know i do. I have friends that are so carefree and have a relaxed attitude and it frustrates me! But you learn to live and somethings are not so worrying!!