There is a great amount of things out there for doing this:
-) Arduino (arduino.cc): produce open source electronic gadgets, often DIY and customizable (might spark his interest, as many gadgets are also fun to use: e.g. tv-b-gone (only building, but fun to use)
-) Lego (http://mindstorms.lego.com)
Very easy approach to programming especially designed for children
-) Scratch programming Language (http://scratch.mit.edu/)
Easy to learn programming language especially designed for children (Age 6 and upwards)
and btw: don't forget your own motivation
If you are willing to teach him in an interesting manner (and in my experience kids can be motivated to take a shine toward anything if it's done with a great approach), then i can even imagine that he might have fun playing around with html, setting up just4fun websites (just don't allow him to do so on the web ;).
Happy hacking :)
I loved that. I honestly didn't get much beyond that, but it was pretty cool to make the screen change.
Scratch is probably good for that. That said, I'd probably start with a language like ruby or python if I was going to teach somebody 12 or up, simply because he can continue to learn it.
"Do you want to learn how to program a computer?" was all it took for my nephew (9) to start.