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Well it's not a C# but a C++ recommendation I have and that's [Programming Principles and Practice Using C++] by Bjarne Stroustrup. (Inventor of C++). It's I think a very good book with a lot of colourful image's and graphical explanations, and as it say's on the back cover:
This book is primary designed for people who have never programmed before, and it has been tested with more than 1000 first years university students...
I think it will work for younger aged also, it has a so called drills after each chapter to establish practical programming skills and in general is a good book to learn the C++ basics.
At 13(ish) I had picked up Sam's teach yourself c++ in 21 days. I wouldn't underestimate the ability of someone that age to pick stuff up. I think a lot of the beginner stuff is too trivial and patronizing.
You cannot argue with agile people so just take the extreme approach and shoot him.