Download a free IDE with good drag/drop support and let him play with it.
Show him how to drag/drop items and run the application. If he has trouble trying to do something, help him out. Don't try to explain all the technical details unless he asks for it. Kids usually have a short attention span, and are more interested in instant results than doing something for a long time with no visible results.
If he stays interested, then you can start trying to teach him a little more about programming. 9 years old is a bit young to be programming applications, but I suppose anything is possible.
I actually recall playing around with some programming application when I was extremely young too... INKEY or something. It kept my interest for a few days/weeks then I moved on to something else. Although now that I think back on it, when I decided I wanted to work with computers and started learning how to program years later, I found a lot of the concepts easy to understand thanks to that early "game" program.