If he's reading at a 7th-grade level, then he should probably be okay reading most adult books.
I've heard said, though never really investigated, that most popular books for adults are written at about a 10th-grade level. I just saw that
A Brief History of Time is on the NY Times Best Sellers list. I've never read it, but if it is written to be on a list like that, the reading can't be too terribly difficult. It's an older book, though. Other than that, I don't know anything. I read books on history, politics, and religion. I just finished a book on coal and am working on another about the industrial revolution and another about the
Great Divergence.
If he's into geology, I had a few books on identifying rocks and minerals as a kid. They weren't really something you could just sit back and read, but they were good references and presented many opportunities for teaching myself about the rocks I found outside.
Really, I'm impressed that he is reading at such an advanced level. Way to go for him! Most 9-year-olds are just taking that step into using reading as a tool for learning; he is clearly lightyears ahead of his peers.
Anyway; best of luck and Merry Christmas!
Jon
Love your enemies!