How good's his Maths?
If he read something like a short history of time and found it an Easy read I could recommend "The road to reality" by Roger Penrose.
This is a serious book! I'm still working through it myself.
It aims to provide a basic mathematical understanding of modern science from black holes to string theory. It starts with a level concepts like complex numbers and starts from there.
It's not a text book but fills that gap between text books and true popular science books .
This is also a very good book
http://www.worldscibooks.com/physics/5088.html
Not much maths but definately up a notch from your normal run of the mill "this is a quark" book.