Reddit mentions of The Math Book: From Pythagoras to the 57th Dimension, 250 Milestones in the History of Mathematics (Sterling Milestones)

Buy this book http://www.amazon.com/exec/obidos/ASIN/1402757964/cliffordpickover

Try look at numbers from an aesthetic point of view besides just logical numbers.

Number theory and such will help because it opens a world of mystery and you begin to realize just how important numbers really are.
http://en.wikipedia.org/wiki/Number_theory

Geometry is also really cool http://mathworld.wolfram.com/topics/Geometry.html

With computers math has become very visual which allows for a greater experience, when you come across a mathematical concept try and Google an image for it, for instance Google "prime number visualization" hit "image search" and your off to the races with incredible stuff.

When I was that age, I used to really enjoy reading books by Clifford Pickover. He has many books, and there are many reviews on Amazon that can aid you in choosing one that you think would be good for your brother-in-law.

Side note: I had not read a Pickover in a long time, but I recently stumbled across The Math Book, which I purchased and quite enjoy. While it may not be the best option for inspiring a 13-year-old, members of Mathit may find it interesting, so I recommend checking it out!

The Math Book by Clifford Pickover is kind of a coffeetable style book, I love coming home from work and flipping to a random page as a starting point to exploring whatever concept is discussed on it. The book traces chronologically through 250 great discoveries in mathematics.

The Math Book. Beautiful pictures on every page, and MOST of the pages are simple enough that a literate six-year-old could understand them. I've never met a person who didn't enjoy flipping through that book.

...so even if your kid doesn't like it, you will ;)