I suggest codeacademy if all he wants to do is learn some HTML/CSS. He can try stuff out on JSFiddle.net by editing he HTML and CSS textboxes.
http://weebly.com is quite good.
Jsbin
I recently found https://thimble.webmaker.org/ which seems pretty cool. I don't know if it's the best, but it's seems pretty good for beginners.
I suggest codeacademy if all he wants to do is learn some HTML/CSS. He can try stuff out on JSFiddle.net by editing he HTML and CSS textboxes.