This guy reminds me of Derek Smart, who threaten to sue a website I worked for once upon a time (Wiki Derek Smart if you don't know who he is) under similair circumsances. Suffice to say, as neither you nor a staff member of the website was the one who posted the post in question, you are not liable for the content of said post, as this is a public forum. He can request you remove the post, but you really don't even need to do that. That said, the content of the original message clearly falls under Parody. It's protected speech.
The only person he COULD EVEN REMOTELY SUE SUCCESSFULLY is the person who posted the original message and you are under no obligation to reveal said details unless he can get a court order for it (and he'd be laughed out of court when he tried to get one). Even then, he'd have to sue your server provider, not you, to get it done.
In all honesty, even if this does proceed to the point that he actually files suit against you, considering his history (and the fact that your not the only website targeted) you could probably get this dismissed as a nuisance suit.