Perhaps approaching from the point of view of programming as a means to end might be helpful? If they have any hobbies or academic projects that could benefit from automation or using computers, that might be a good starting point?
For example, when I started studying engineering, it was great to see solutions of differential equations and ways to solve them on computers. Others have suggested games which could be great if they enjoy gaming and want to know more behind the scenes. Basically my approach would be to find a problem or two and use programming to show that computers can do wonderful things once we learn how to interact with them in various ways.
Yes, I completely agree. One thing I'll add as an example; I used programming as a way to learn about mathematical concepts too. I made a program to render an image of the Mandelbrot set, which helped me learn both some new things in JavaScript (what I was using) and mathematics (complex numbers and limits). You'll probably be able to find a similar sort of project for your son.
For example, when I started studying engineering, it was great to see solutions of differential equations and ways to solve them on computers. Others have suggested games which could be great if they enjoy gaming and want to know more behind the scenes. Basically my approach would be to find a problem or two and use programming to show that computers can do wonderful things once we learn how to interact with them in various ways.