If he likes Philosophy, then I highly recommend "Godel, Escher, Bach."

Its highly accessible for most mature readers and its written along the lines of "The Art of Motorcycle Maintenance." and Plato's "Socratic Dialogues", but covers much more ground in areas of Math, Art and Music by looking at the patterns in knowledge. Historical figures in each area are introduced and their ideas explored.

I got a copy when I was 15 and was hooked.

http://www.amazon.com/Godel-Escher-Bach-Eternal-Golden/dp/0465026567

http://en.wikipedia.org/wiki/G%C3%B6del,_Escher,_Bach