Meh-- My DD was avid about Magic Treehouse books for a while-- she gobbled them up like potato chips. Two or three an evening/afternoon.
I don't think that they harmed her any, since she just (for fun) polished off Jane Eyre at 13, and this is fairly typical for her. She's not a book snob, which is fine by us. She also still loves graphic novels and comics. Again-- fine. She loves to read, and it's good for her, so why not?
Beverly Cleary has a number of really delightful books at about that same level (Ramona)-- The Mouse and the Motorcycle is a great set of books. Shel Silverstein's poetry anthologies are also a lot of fun at this age, and I'd encourage you to look up Hank the Cowdog and other Roald Dahl books, too.
Once he finds what he likes in terms of genre or content, my advice is to let him decide what he wants to read and be fairly hands-off about it. Let him own it.

That's the secret to making a kid a reader, IMO. Help them until they find what they can love, and then get out of the way.
PS. We LOVE the Captain. And Doctor Dog, which every member of my family can STILL recite from memory. LOL. Of course, this may tell you something about us. But maybe not, too. We also adore Whitman, Hugo, Dickens, and Shakespeare. We're just not judging print by "good" versus "bad," only by what we like and find interesting.