Hm. I first caught my glimpse of HTML/CSS when I was around that age. I had an older friend who introduced me to Hack This Site and the Matrix movie. That's what initially piqued my interest to dig into programming (HTML was the first topic recommended by HTS).
So, is there a particular reason he wants to learn? If not, maybe watch the Matrix or similar with him to bring that interest to new heights. Try out Hack This Site (it's still around). There's nothing better than experimentation. Whatever his other hobbies are, make something around that. It could be a simple static page about Spongebob, or something. He can take that skeleton of a site and swap out the elements for another one of his interests (i.e. Change the Spongebob images, text, etc. with Invader Zim).
From there, he'll probably continue learning, especially if he's anything like me, holding some of the same interests since 10 years old.
