A few options that I, Hubby, and my kid have enjoyed at that age or close to it:
Geronimo Stilton series (about a mouse)
Enid Blyton books - Fabulous Five series
Hardy Boys
Magic Treehouse series - there are both fiction books and companion reference guides (e.g. book 2 is “The Knight at Dawn” and there is a companion guide on Knights and Castles)
Farmer Boy by Laura Ingalls Wilder - this is about Laura’s husband, Almanzo, growing up on a farm in New York State
The Five Little Peppers series - these would probably be more available as e-books. They’re about a family of five children, three boys and two girls, and their adventures.
Little Men and Jo’s Boys
Adventures in Odyssey series - Christian fiction
Sigmund Brouwer books - many of these are sports-focused and they are written by a Christian author
Kids at the Polk Street School series by Patricia Reilly Giff - these are older, but he may enjoy them. They’re about children in a Grade 3 class, I think it is. There are a couple of companion series, such as New Kids at the Polk Street school (about the younger siblings, who are in Kindergarten), and Polka-Dot Private Eye, but these are more about girls.
Bailey School Kids series - this is a humorous series about children who suspect that teachers and staff at their school are various sorts of monsters. For example, one book is titled “Martians Don’t Take Temperatures”. (All of the titles take the form of “[Monster(s)] Don’t (Doesn’t) do [X]”.)
Animorphs series - these are about a group of young people who are given the ability to “acquire” animal DNA and then “morph” into that particular animal, in a quest to save the world from invading aliens.
Nate the Great series - I enjoyed these when I was a child. They’re about a child private eye.
Encyclopedia Brown series - these are about a 10-year-old boy who solves mysteries, both as an assistant for his father, the chief of police, and for the neighbourhood children. Readers are given the opportunity to see if they can “solve” the mystery, using the clues provided, and then the solutions are at the back of the book.