[russellallen]

Yes, it is possible that a court would construe the action of placing this public domain mark on a work as evidence of an intent to freely and irrevocably licence the work to everyone without restrictions. I wouldn't advise a client to rely on it where money was at stake.

However the term 'public domain' refers to works where the copyright has expired or where the work did not meet the requirements to attract copyright to begin with.

This is the meaning used by the CC. They say in TFA that "The Public Domain Mark in its current form is intended for use with works that are free of known copyright around the world, primarily old works that are beyond the reach of copyright in all jurisdictions."

His website is not in the public domain in this (legal) sense.

I'm assuming exactly the same as you - that he wants to make his website as free as possible. The way for him to do this is to put his website under CC0 or CC-BY.

[ars]

That's not the only meaning of "public domain". You can also do it actively: http://cr.yp.to/publicdomain.html

The reason CC0 exists is that placing something in the public domain is not possible in all countries.