His school runs a "reading bingo" challenge offering various levels - i.e. level one has them read a book with at least 85 pages, a Dr Seuss book, a book of poems, a joke book, etc, and then increases to reading Dahl, Diary of a Wimpy kid type stuff and so on.
The most recent level has one of the items as a book with at least 350 pages. We got him one of the How to train your dragon series books with 400 odd pages and he claimed to have finished it in about 3 days - when pushed a little (he reads fast, but not quite that fast) he admitted that he had skipped all the pages with text only...
Surely this isn't normal?
How about you try "The Invention of Hugo Cabret". It's a great book and it's "long" but still has many pictures and not too many on each page.
And it's a Caldicot Medal winner.
As to is this part normal. I believe this a common asynchronicity in early/gifted readers. Teacher & parents tend to see reading level and say.. OK my kid should be able to read Harry Potter. But while my DS could read the "reading level" for HP in 1st grade he was NOT ready for that level of book. And it's why he spent most of K/1st reading non-fiction.
Reading level often is calculated basic on complexity of the words/vocabulary. I bet he finds Hugo Cabret an easier read even though they have very similar reading interest levels and reading levels. My experience was when you tested my son against short passages my son read extremely well he just wasn't ready for the task of reading quietly long stretches of just text. I don't think this is that unusual.
As I said in my previous post I would back off a bit on the "length" requirement of his fiction reading. Just make it fun. Has he tried the Geronimo Stilton books? They were a huge hit with my daughter when she was in 3rd grade. And I believe the use of multiple fonts per page helped. And Graphic Novels are an excellent choice. It's an area of books for children that's really growing and I think it particularly helps with kids like this.
Edited to add. Do you read with your child? I read with both my kids until they were older than your son. By K we were chapter by chapter reading a book together nightly before bed. (Often something 2-3 grade levels above their solo reading.) This is a great way to teach how to comprehend a longer book and I'm surprised how many parents stop reading to their child once they can read to themselves.
Good Luck
