You have so much fun ahead of you. You're also smart to think about content as much as reading level. It's tough when they're young and can read well but are not ready for the content of the "reading level appropriate" books.
First, do go to your local library and ask to see the children's librarian. If there's a children's section there should be a children's librarian. Talk to that person for recommendations -- it will totally make his or her day because this is what they live for! Also be sure to talk to or e-mail your son's school librarian; again, this is what they do best. And librarians are aware of the content-versus-reading level issue you face.
Think about his interests. Is he into fiction and make-believe or might he prefer nonfiction, factual books, and "list" books? Very generally speaking, a lot of boys through grade school often prefer nonfiction books about subjects they like, whether it's ancient Rome or cooking or a sport. (I check out books at the school library for 4th and 5th grade and see all those topics are very popular with boys, while girls check out more fiction, though certain fiction series are huge with boys.) Take him for a long stay at the library and let him really ramble and be sure he hits nonfiction as well as fiction sections.
Don't forget magazines too! He might like National Geographic Kids, Highlights (has both fiction and nonfiction), Cobblestone (kids' history magazine) and much more -- you can get them for free in libraries!
He might like books by Andrew Clements, who writes "school stories" that are terrific; read them first to see what you think. Also try Louis Sachar's very funny "Sideways School" story collections. Encyclopedia Brown is not the most advanced reading level but is great for making kids think through the mysteries that Encyclopedia solves. In fact any kids' mystery series might appeal to him because they will make him work while he reads; A to Z mysteries by Ron Roy and "history mysteries" by Carole Marsh are other possibilities that are popular. (We have adored Caroline Lawrence's Roman Mysteries series but it may be far too scary for your son, since realistic Roman-era violence is throughout the series, plus, you would need to explain a lot of the historic background. But read those when he's older, if he likes mysteries and history!)