Aid me, O Internets

[adelynne]

I need a couple chapter book recommendations for a boy, age 7.5. His dad says he's reading at about 5th grade level, but nothing really "substantial" (Wimpy Kid, Captain Underpants) - he'd like to see him get into a bit more substance.

I'm guessing there's tons of books out there for this age range, but as I missed it in written English, I'm hoping for ideas.