I have a five year old that questions every thing he sees now. I limit the things he sees on tv - especially with what is going on in the world today as this is too much for a little mind to comprehend.
Every child suffers stress of many types. Your son could be processing events that he has gone through from the day, things he has seen, or heard. He may have heard things that you have talked about with others or saw things on the news as well. There are so many things going on that can affect a mind at that age that yes, I can believe it can do what it is.
I am a CMT, so I deal with this as my work. There are options to help. With my son, I am able to provide the things he needs to help him learn to properly shut down and process. Meditation, yoga, massage, reflexology as well as just talking about what is on the mind or what happened that day... if this doesn't help alone - then yes, you should take him in to see his doctor or a professional counselor. Or do a combination as this will help for later in life to help him learn to deal with stressful events. Everyone needs to know how to relax and unwind to stay well - physically, emotionally and mentally.
Update: After reading some responses... I do not recommend the story playing in the mind as sometimes even I suffer this on sleepless nights and I also have an active imagination. I have had less sleep on those nights of running story lines than if I left it alone. Those are the nights that I ended up playing a meditation cd and finally feel asleep. However, I do recommend writing in a journal before bed as that is another tool I forgot to mention (as I was up late responding to you~).
Be positive and patient, he feels your concerns and I am sure it will work out for you. :)