It's great to see you encouraging your son! Arduino might be a good investment, it's a good mix of software and hardware interfacing. You can buy different kits and you'll end up with a PCB board that plugs into the computer via the USB which you can program with the software that comes with it. It's really remarkable stuff and relatively cheap these days (lots of demonstrations and applications on the internet and youtube). I've never used it myself, but i believe the software is almost like C++, but wrapped within something a little more user friendly and so it's good for people with no programming experience all the way up to advanced. I'm sure Arduino users here will be able to give advice on which kits are good etc.
Megamox