Dd is 7, and she read the Harry Potter series the summer after 1st grade.
Dd is very much into are historical fiction. There's a series called "Dear America" which are fictional diaries of young women at various points in US history. There's a spin off series called "My Name is America" that features young men. If your son likes war books, he might like these as significant periods in history tend to talk about war.
There's a series called "My America" that has a mix of male and female characters. There's on on the Oregon Trail (3 books) that's got a male character. There are some about the Civil War and several other major periods too.
Dd is also reading the Warrior Cat books. They're a little more violent that I like (the cats have battles), but dd is OK with them.
Have you read Beverly Cleary? I love her books. My kids are re-reading The Mouse and the Motorcycle series. The Beezus and Ramona books and the Henry Huggins books are good too.