My DS 8 worries all the time as well. Since he was small, he has expressed his fear of dying. I don't know where it comes from, but he brings it up at least once a week. He has other anxiety as well, like if people like him, if he's going to get sick at Disney, if the dog is going to run away, etc.
Finally I asked him a few probing questions and think I pinned it down to my DHs TV habits/ He has the news channels on all morning, especially when DS is getting ready for school. If you've watched the news lately, you know it's about child abductions, plane crashes, natural disasters, etc. No wonder he's afraid. I asked DH to turn off the news; hopefully he's doing so, but it'll take a while to get undone. In the mean time, when he has a fear, I ask him to tell me about it and then we break down how realistic it is...usually it calms him, at least for a while.
Obviously at your child's age, that's probably not it, but that's the age they start to realize that bad things really do happen...it's natural.