Comment Mathematics of Choice (Score 1) 238
Get him Ivan Niven's The Mathematics of Choice (How to Count Without Counting). Most computer nerds never heard of this one, and may not get the relevance, but I think it is totally appropriate for young teens. I read it in high school when I was 14 or 15. And yes, this is the best recommendation you will receive today.
It's a basic, extremely well-written combinatorics book, so you can read the book yourself and create interesting programming problems for him for reinforcement. Don't deny your nephew this AMAZING opportunity! :)