Have you tried the Encyclopedia Brown series? I ate those books up as a kid, and I was the type of kid who mainly read comic books.

I will add, in favor of comic and other "non-serious" types of books, that two independent testers placed me as reading at a 10th grade level when I was 9, so I must have been getting something out of all those Uncle Scrooge stories.

Does your son like science? My eldest loves to read science encyclopedias for kids. Also, what about magazines such as Ranger Rick, etc.?

I think that the length of a chapter book can be off-putting to some kids, especially if they aren't used to reading them. Books with discrete, bite-sized pieces can be the start of a good pathway into reading longer things.

Val