Not having a clue about how to do these things, I think your answer lies in your own experience—you had to go through the challenge of working it out yourself—sometimes it was hard. I don’t know if there are shortcuts to this kind of understanding a subject—any subject. He has to figure it out for himself. You can support him though—and maybe show him where to go, or what to look for. But the hard work—no substitute for that. And that’s also the fun of it. And the feeling of accomplishment.

I’d suggest to give him a small project that he can execute in parts. First a static webpage, top-to-bottom without styling. Then teach him how to add some styles to it (i.e. change font-size, font-family, color, background, borders, forms perhaps). After that I’d move on to positioning with CSS, e.g. let him add a sidebar. Then, have him add some interactivity using CSS (a guestbook is a very popular starter). Finally, tell him it’s just the tip of the iceberg, advice him to try playing with a PHP framework (great for learning best practices) and come to you if he has questions.

Of course, every step would consist of 1) some basic instructions, 2) have him have a go at it and finally 3) checking what he did and give tips on what he did wrong or could do better.

Oh, and perhaps some words on where to find help (i.e. the PHP manual ;-)) is a great idea as well.

First, the concepts. Explain the difference between content, style, and queries. Then, let him look at the code of actual simple sites and try to duplicate the idea. I think it’s good to realize what affects the elements. Show him what an unstyled page looks like: it’s just text. Now, how do you make it snazzy? The tutorials that go with WestCiv’s Style Master are excellent and free. I personally use Panic Coda, a Mac-based dev and design app that is dreamy.