Ruby would be my suggestion -- though Python has a great base of education material and would also be a good choice. The benefit of Ruby is that the language itself is very forgiving and extremely 'readable'. There's an app called 'HacketyHack' that you can use to teach Ruby, and it lets you create windows and interfaces too, so it's not just console based hacking. It'll need porting to RaspberryPi, but I'm sure somebody will do that (if it doesn't already build on ARM). Check it out: http://hackety-hack.com/
I'd stay away from Java (won't work on RaspberryPi anyway -- I don't think the Java SDK works on ARM), and C/C++ (those are generally reserved for games programming and operating system stuff these days). I'd also stay away from PHP, unless you want him to make basic web pages (but then Ruby and either Rails or Sinatra would do a better job).
Basic is fine, but there are very few modern basics that let you do exciting (or interesting) stuff, so it might just seem a bit flat. Back in the old days, Basic was fine because computers were simple and text on a screen was just fine. BlitzBasic is the exception that springs to mind (it's aimed as games programming), but it won't work on Raspberry Pi.
There's a weird and wonderful guide to Ruby called Why's Poignant Guide. It goes through some of the basics. A lot of people tell me they found it too 'strange', but I loved it. It's worth taking a look anyway, because it won't cost a penny: http://mislav.uniqpath.com/poignant-guide/