Have you considered GAD - generalized anxiety disorder? This anxiety disorder (which is an inherited trait and not caused by life experiences although life experiences can exacerbate the issue) often begins around eight years of age. I might suggest you google phrases as "GAD and children" or "behaviors of anxiety in children" or "anxiety disorders and children" or similar words/phrases.
If you feel that GAD might be the issue, then visit your family doctor re advice (and visiting your family doctor does not mean medication). The material on-line or in bookstores or your public library should be of help. Hope this helps ....
So he likes school yet cries before he goes, so he is concerned about you , have you any new relationship that may be the cause of some feelings from him ?Where would he get ideas about a bomb, has he been chatting to an older family member ?
I am very careful about what he watches on tv. Probably too over protective. I asked him if everything was ok at school. I even talked to his teacher. Nothing but good things from his teacher.
One other thing what does he watch on TV or the Computer is it supervised ?
It sounds to me as if the problem maybe something is happening at school, it may be a good to speak to the teachers, the fact he cries before school tell you, could be some bullying going on, ask him pertinent questions about other kids at school and friends,he doesnt want to go on school trip is another indication something is going on .You may have to take a day or so off to go check out what is happening its not going to get better till you do . Does he see his Dad regularly , visit with him , maybe he would tell him whats going on