My kids are older now, and I can't remember specific titles, but I would search for books about Fibonacci; I think we found several good ones. There was a math section in the children's library that had several books like this. You might also search for things on fractals, even snowflake books might have some of what he's interested in.

He might also enjoy Vi Hart's YouTube videos, I know there is one on Fibonacci, but many are about patterns (maybe not so much about nature, though).