We've been using this one: https://www.w3schools.com/html/ He's had no trouble at all following along and using their tooling. I've been mostly hands-off.

I'm about to graduate him from "do it in just the browser" to using GitHub pages. That way he can have an HTML site that's "his" and start putting the pieces together.

Just so you know, W3C is the main standards body for the web; w3schools is an unaffiliated privately-run web site with an intentionally confusing name. A much better general web resource—certainly for any intermediate-or-above learning, but probably also for beginners—is MDN[0].

[0] https://developer.mozilla.org/en-US/docs/Learn

You may also want to consider a Droplet for his next website. He will learn about the plumbing.

That way, when ever he is filled with self-doubt and needs reaffirmation, he would be able to put things together from the ground up and regain confidence.

What is a Droplet?

Edit: seriously. Non native speaker here, I have never heard the term. Googling shows: https://www.digitalocean.com/products/droplets

So a VM from this vendor is meant? Why?

VPS hosting at Digital Ocean, available for USD 5 per month