[math-fun] Learning computer programming for a bright 13-year old ?

My very bright 13-year-old nephew would like to learn computer programming (from scratch). He's quite familiar with using software, especially having used Midi music software for several years now. At this point I don't think he has any specific goal of building a particular kind of software; he just wants to learn what computer programming is all about. Has anyone had success with a particular language and/or book in similar circumstances? All suggestions are welcome. At this point I'd like to give him something simple (so probably Java is out) but not too simple (so maybe Alice is out). But my mind's not made up about these, either. Many thanks for your ideas. --Dan