He probably doesn't have to work as hard to conjure up the pictures in his mind if the pictures are already there. I also have a strong preference to re-read fiction I've already read rather than venture into new fiction. The initial climb is very unpleasant for me.
I have no idea if I'm dysgraphic, but I have always known that I was a slow reader. What you describe sounds so similar to me that I can't help but wonder if I am. My DS has some characteristics that have us on the lookout for dysgraphia as well. He doesn't seem to have the same problem I do with fiction, however.
CoastalMom after reading the above I thought of a few different idea's for things to try.
-- When listening to an audio book have him read along on the printed page. (If possible.. we all get carsick so in my family this doesn't work in the car.) My older daughter had reading LD's and this was a recommendation by the educational therapist that worked with her.
-- With a new book or series, try reading out loud together the first few chapters enough to get that picture of the book in his mind. Either you read it or trade off reading different pages or chapters. See if you can get him to finish the book silently on his own.
--Try having him watch the movie first. (HP, Holes, there is a lot of options to chose from.) I don't usually recommend this but it might help him with mentally setting the picture. Maybe challenge him by making it a game to find all the ways the book & movie are different.
Your son isn't the only kid his age who balks at chapter books. I can relate my son who taught himself to read at 3 was uninterested in chapter books (without pictures) until about 3rd grade. But he could and would read non fictions books like crazy, and fiction that still had pictures was fine. I became quite adept at finding books that would challenge & interest him that weren't long chapter books. When he was younger I just took this as being an asynchronous of gifted learner that while he could read words he wasn't ready for whole books. Once I finally had him tested recently it showed that he has a low working memory & processing speed.
On the other hand in K he loved books like Captain Underpants, The Cheese Books (Geronimo Stilton) and other books that had a lot of words but still had pictures. I don't think this is unusual. The genre of very short "early" chapter books without pictures is fairly recent and IMO not a necessary stage to go through in learning to read. You might check out graphic novels is a huge emerging market and there is a lot of variety there. My son only really took off reading fiction after inhaling the whole HP series long after having seen both the movies & listened to the book on tape.
