Your son sounds like my son! A few years ago I went through the same struggle. Bright kid, reading way above grade level, but spends all of his time reading Dogman and Captain Underpants. Tried a bunch of books that I loved, brick wall. He'd read a page or two and then back to the Captain.
By now, he reads "text" books all the time. Not necessarily the books I'd prefer, but the complexity level is increasing, and honestly the biggest problem we have is that his reading level is higher than his interest level - which is a major part of this problem. A kid that can read at an adult level still won't want to read those books, because they won't be interested in the themes an adult would. (My mom went through a phase where she had me reading the great Russian novels, Dostoevsky etc., as a ten year old. My reading level was fine for them - I had no issues with vocabulary or length - but I wondered why they were so boring. Guess what: it's because I was ten, and didn't understand any of the themes.)
We got there by doing two things, plus mixing in some patience.
We bought a bunch of more complex graphic novels. There's a lot of really high quality literature out there that comes in graphic novel form. Check out First Second press, for example; they have a lot that's 9-year-old appropriate that's complex and exciting. Amulet is pretty good. The 2020 Newberry Award winner? A graphic novel, "New Kid", which is extremely complex but also extremely readable. Kwame Alexander is great as well.
This also included some graphic novel adaptations of books - one
example that worked particularly well for us was the Wings of Fire
series; they have the first three or four in graphic novel form. He read that, and then when he got to the end he wanted more - so he started reading the text versions. 17 books later, we're finally out of that, and he started to look for other things on his own!
We read with him as you do. Harry Potter in particular worked well this way. We read a chapter a night (at first) and then a half chapter (when they got insanely long). By the time we were in book 4, he was taking the book to bed and reading the rest of the chapter most of the time, and book 7 I had to frantically read at night on my own because he'd read two or three chapters after I finished my part, so I could be caught up (I hadn't read them yet myself). This won't work for everyone, but for children who get caught up in the excitement, it can be very effective.
None of this is instant of course; patience is required.
We also did something slightly different with Narnia, during the spring Covid out-of-school mess. He had to do a certain number of school work activities in order to get his free time. One of those was reading; we decided together that Narnia was the series he'd read. So he'd read a chapter as one of his works. Similar to Harry Potter, that lasted a few books, and then everything after Dawn Treader [reading in the original publication order] was read in a few nights on his own.
I think graphic novels were a great transition for my son, because he has trouble visualizing things in his head (like I do). Until he developed the ability to more effectively translate words into what they mean visually (or, like I do, developed the ability to understand but mostly skip the scenery, since I literally can't picture it) the word versions of books were too frustrating to read - but graphic novels helped him out by adding the visual stimulation. It's just a matter of finding good, complex literature in that format, which fortunately isn't very hard these days (thought it's quite expensive if you don't have a good library ...)
One thing I want to add here: don't expect him to enjoy the books you enjoyed, and don't push them too hard. He might like some of them. He might not. He's not you! That's a mistake we all make, in part because we want to share the joy we had with our kids - forgetting that they are their own people. The way to get him to read the books you read is to make them available. Kids that enjoy reading will tend to eventually read the books they have available - so they'll read your favorites, eventually, and either love them, or not.