Just to throw my 2 cents in here... (as a Python developer)
Used with Qt and some Python libraries (such as PyQt) you can build entire cross-platform desktop applications.
Its used a scripting language for Autodesk tools (3DS Max and Maya), you can make plugins for these.
Its great a general-purpose scripting language, with very powerful libraries. I've written scripts to dynamically generate images based on data input. Usable on the Pi quite easily too.
Its great for learning the basics of programming in an easy-to-understand way. That said, I'd always advise branching out once you've got one language under your belt - JavaScript and C# are pretty standard these days.
As a final aside, its on the new GCSE's computing courses as standard, so its worth picking up as it'll come useful at that stage (assuming he's not at GCSE-age yet).