Minus the ADHD, he sounds just like me when I was 11. I'm still scared of the dark (I definitely did what your son does with reliving books) and heights and clowns and many other things that frightened me as a child, mostly because I never dismissed them as nonsense. They make perfect sense to me! But, I'm not so scared now that I can't face them.
What helped me as a child was having cats. They were something known that existed in the house all the time, so that when it was empty and dark, I knew that most sounds were the cats (it was when my parents took the cats away that my fears redoubled until we got a dog). It also helped to have a brother that would go on adventures with me in the dark sometimes, so it was less scary when I had someone with me and I got to explore what I couldn't see. Having my parents help wouldn't have helped very much at all. There was something far more effective about having a friend (my brother) or an animal to go with me in times when I was scared. I think it's because I didn't have an authority figure following me around.
Anyway, it's taken me a long time to grow up in the sense that most people expect. I've always been rather self-sufficient, but socially, I've never been very independent. Your son may take a lot time to reach that point as well. I think he'll need lots of reassurance, I know I did/do! A lot of the things that are considered socially normal I question as to whether or not it's "okay" for me to do that.