I recently watched a friend in his mid 20's try to install some poker software on a new laptop. He's been playing poker online for years.
Anyway, to get the software he clicked IE which brought up the default home page (AOL). In the AOL search box he typed 'google' and selected the first result. Then when google loaded he typed 'poker stars' in the search box to get to the correct site.
He's in love with the idea of writing a game. He might not actually like programming.
So, you need to start with something simple that will get him immediate results. Which really means HTML these days as everything else needs umpteen objects before you can see any results.
If he takes to it, you can then move to CSS and javascript. And if you get this far then the leap to actionscript or python or whatever isn't that big and you'll know that you have a good student,