This is my first post here.
To get your son interested in programming is not the question; he already is. The task is to get him to actually program! I feel that the best way is to pick a language (I like C++), get him SIMPLE books (I like the "For Dummies" books myself), and then set him tasks that you applaud when accomplished. The applauding is VERY important!
I suggest graphics oriented tasks such as:
Make a word move on the screen.
Make a ball move up and down.
Draw a square with corrected aspect ratio.
Make a ball that bounces around the screen.
I suggest Text oriented tasks such as:
Make a quiz using Data statements.
Make a math quiz with random questions.
Make a "Mad-Libs" game with user inputs.
Make a scrolling "Times Square" sign.
Limit his access to libraries of functions for now. Show and encourage him to to create his own library.
Remember: Your son may not actually "take" to programming. He may not be as fascinated with the feeling of power we get from controlling a machine as the rest of us are.