One of mine was very into Captain Underpants - I think there was a caveman series by same person/people too.
He liked the Geronimo Stilton books very much - I think he moved on to those around that stage. They are adventure type books - a mouse goes on to help friends around world, so they are kind of educational in that respect, but he's always getting into predicaments. Not too serious, and there's a nice series so if he likes one, the school will have a ton and library.
He also liked the Roahl Dahl books - those are pretty silly, but a bit more advanced than Captain Underpants. Think of Chalie and the Chocolate Factory.
He loved the Hogwarts series . Harry Potter.
Another one of my kids loved the Guiness World Record books (I think they were big encyclopedia type things for kids) - never really loved reading so much as looking at silly/cool pictures and reading up on where and when.