My son ( 10 ) wants to build his own website and wants to do it using HTML/CSS.
He doesn't want to use a website builder as he wants to understand how it works fundamentally.
I set up a netifly hosted html file to start out with and have taught him the basics of HTML and CSS and how to link to other pages.
Any tips to teach him the fundamentals?
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.