I think it might be worth taking a pause here and realizing that the "problem" here is that a 9yo who consumes reading material like mad is not getting enough fiction. Is it really a problem? And if he's reading nonfiction at a high lexile level, should we really worry that he sets it aside for a bout of Geronimo Stilton?
GeorgeC's description of his reading process is similar to mine, though I suspect it's happening at a speed much faster than the spoken word, these days (I've looked at the process for speed-reading, and honestly, I don't understand why anyone would do that to themselves). And I'm definitely not an undiagnosed dyslexic. I do find reading to be quite comfortable, and I perform the same reading process whether it's non-fiction or fiction.
Dialing back the calendar, though, I was probably 9 or 10 when I started reading fiction in earnest. Before that I was avidly consuming non-fiction, and when I exhausted what the school library had to offer on that, I turned to comics and joke books.
What separates a fiction novel from all these other sorts of reading material in the discussion so far is this: page after endless page of dense text, occasionally broken up by a chapter header. These other media are like short sprints across the playground; kids do these all day long without even thinking about it. Then you hand them a novel, and they see Mt Kilimanjaro. It's an endurance read where they've trained for sprinting, and each step seems to get them no closer to the top.
And so the solution is... endurance training. This is basically what the industry does by providing early novels which are thinner, have larger text, include illustrations, and use more frequent chapter breaks. I recall my DD10 constantly updating me on her progress with more adult-like novels when she first started reading them on her own, whether it was announcing what chapter she was on, or showing me the thickness of the book relative to the placement of her bookmark. She needed that reinforcement to let her know she was making progress, and confirm that she could reach the finish line. And as she did so, I remembered being in that exact same position.
Because if you're climbing Mt Kilimanjaro, at camp and tired at the end of a long day's hike, you want to know, am I ever going to get to the top? So you look down below to see how far you've come, and you look up above to see if the end is in sight, and that gives you the motivation to start up again tomorrow.
So, while he may be cognitively ready to tackle the entire Harry Potter series, it might be better to give him Captain Underpants today and let him work up to it.
