I'd recommend a basic language, whoever suggested using 'C' must have been trying to put him of. There is a demo version of a programming language called Blitz Basic which has a very supportive community and comes with a number of example pieces of code for you to crib from. What more it allows manipulation of 3D objects from very simple commands, which is probably a little more gratifying than seeing "hello world" in a terminal window from C. Here is a link to the Blitz3D product: http://www.blitzbasic.com/Products/blitz3d.php ;) there is a product called Blitzplus which is a simpler version (no 3D element), both the products have demos available to download from here http://www.blitzbasic.com/Products/\_index\_.php Lego mindstorms can be gratifying, but I'd wait until he is 14 or 15 before opting for this..... I've used it in conjunction with a HNC in BIT and trust me, Blitz3D is a lot more gratifying for the level of effort that needs to be exerted.
Keep on coding