I second the Pratchett suggestion as easy fun reading.

When I was a kid that age I was reading Robert Heinlein books, and though they are dated, they are not so deeply written as to be inaccessible to a child. Adventurous stuff to me.

SOmething else I liked were the Tom Swift books. The old originals are probably better read as an adult retrospectively, but the TOm Swift Junior stuff was adventurous. I haven't look through the youth shelves lately, but for all I know there are Tom Swift III books. I think Tom appealed to me because I was a "science nerd" kind of kid, and Tom was all about using science and technology to get where he wanted to go, he invented things.

GO to the bookstore and look in the youth section, and there are a whole pile of "series" books. We had Hardy Boys and Nancy Drew along with Tom Swift, but I know ther are all manner of series out these days. Our kid liked the Goosebumps series for a while. With a series, if he likes a book, there are a whole lot more with the same characters waiting for his developing interest.

Arthur Clarke was a favorite, and don't overlook short story collections.