I know this is late, but I was very similar to your child at that age. I've since been told by a therapist that I may have been "born anxious" (I found that not helpful, but she would not go into detail on what she meant or why she thought I was that way). Anyway, I was always in gifted classes and very imaginative, so I had thoughts that I really couldn't explain.
For example, I remember hearing that an alligator had escaped the local zoo and been recaptured in the pond where I had swim lessons. The water was murky and I HATED going to lessons after that because I was sure the alligator had laid eggs and there were alligators in the water where I couldn't see. But I couldn't explain that to my parents or teacher for some reason.
Also, I hoarded rosaries and crosses. My dad thought it was funny to tell me vampire stories, and he would never, ever, ever admit they were made up. My mom didn't know this, so she tried to "cure" my fear of the dark by making me go upstairs without a light, etc. Again, I couldn't explain for some reason.
Or my grandfather said if I swallowed an appleseed, a tree would grow in my stomach. Yeah, i worried about that too!
I also just didn't like certain smells or textures. For example, I still really don't like to touch flower petals. No idea why! It took me a year to learn to ride my bike--I just didn't like the way it felt.
A lot of this just fell by the wayside--I outgrew it, or the circumstances causing it changed, or I learned to manage it.
I don't think it would hurt to have your son evaluated, but please make sure he doesn't feel like there's something wrong with him. I know a lot of my inability to communicate was because I felt it was shameful or lacking or wrong to be worried about things. Also, the more patient and supportive you are, the more you listen at his pace/level, the more likely you are to find out if something's going on--like someone telling him stories or being hypercritical or bullying him.
Also, there is a great book called Raising Your Spirited Child that talks about how some children are just more sensitive than others. I'm actually not a huge fan of parenting books, but this one is really common sense and even with all that I went through, I find it really helps me with my daughter, who is (surprise!) similar. We just went through a year of anxiety about her beloved lava lamp because another kid told her it would explode and send lava all over her room.....
I hope this helps. I know i baffled my mom, but I know that with love and patience and maybe a few professional tips, he will be fine!