If your son truly has an interest in the fundamentals of mathematics as a whole he might enjoy a fantastic book I read recently called "Mathematics: The Science of Patterns", by Keith Devlin. It's not overly technical, and it covers various topics such as counting, motion and chance, shape, and symmetry, and gives a general history of the development of mathematics as a whole. Although there are some parts which are a bit complicated, the book doesn't assume specific knowledge of any particular area of mathematics, and I'm guessing a bright kid possessing an elementary education should find the majority of the text comprehensible and intriguing. Any Barnes & Noble, Borders, or similar book store should carry it. Hope this suggestion is helpful.