My experience working as a reading tutor is that boys like non-fiction. Either that or super stupid fiction, like Captain Underpants or Diary of the Wimpy Kid. They don't really relate to fiction chapter-type books the way girls seem to.
Really untrue for my 5yo boy (6 in spring). He loves fiction chapter books and has been reading them for a good long while now. He likes nonfiction, too, but fiction is a huge interest, and definitely not just bathroom humor (not sure he has read any of those). Just wanted to point this out because I think these generalizations about "boys only..." can be a bit unhelpful.
Your librarian can be a great guide, but for now, I'd try him on a whole lot of different stuff, including slightly harder read-alouds that might catch his interest. Some books my DS has liked include the Narnia series, Dahl, some Blume, Cleary, the Indian in the Cupboard, the Secrets of Droon series, Half Magic, Catwings, Kate DiCamillo's books, Franny K Stein Mad Scientist, Toys Go Out, and the Stink series by McDonald.
But don't rule out great, harder picture books, either. They are out there.
