This may be what you want. http://web.stanford.edu/class/...
To give you an overview, it's an intro to programming using Javascript and a little image manipulation library. Each page has a series of problems with boilerplate code that you edit and click a button to run.
Head straight to Week 2's lessons (http://web.stanford.edu/class/cs101/image-3-loops.html) and go through it with him.
Kids find it pretty cool that they can change some numbers and that will have an effect on the picture. I did this with my nieces (8 and 10) just a few weeks ago and they both LOVED it. I was showing it to them to gauge their interest for a totally unrelated reason, and we ended up going over it for about 3 hours in one sitting.
Give your kid little challenges and provide most of the code. They just edit the code. It'll be a while before they add their own lines of code (about 2 hours for my nieces). One of the big points of interest was one of the problems that introduced an "If" statement based on the pixel's X coordinate. We made many changes to that block of code and ended up making stripes of different colors across the image, first vertical, then horizontal, then mixed. They thought it was just the coolest thing.
Then there's code that changes pixel values based on the average color of the pixel. So it's doing stuff like taking a picture with a red stop sign in it, and making the stop sign blue without altering the rest of the image. It's really neat, and it's the kind of stuff they've seen in movie special effects (they'd heard of green screens and I related how similar this is), and it's just a few lines of code.