Vernon Unsworth is suing Musk for calling him a "pedo" in a July 15 post on his Twitter account in an insult-fest that unfolded last (northern) summer after the underwater rescue of youth soccer players trapped in a cave in Thailand.

Last week, a federal judge in Los Angeles set an October 22 trial date, rejecting Musk's attempt to dismiss the lawsuit.

In a response to Unsworth's lawsuit filed on Monday, New York lawyer Alex Spiro wrote that Musk's statements were opinion protected by the First Amendment, and that Unsworth can't seek damages because all or parts of Musk's statements were true or substantially true.

The document doesn't specify which statements by Musk are true.

"The formerly unknown Mr Unsworth remains the utterly undamaged Mr Unsworth," Spiro wrote.

The spat began when Unsworth, who shared his knowledge of the caves that he had explored, dismissed Musk's efforts to help rescue the soccer players.

In an interview with CNN, Unsworth called them a "PR stunt" and derided a small submarine that Musk had built for a rescue mission.

Musk lashed back on Twitter, where he had 22.5 million followers at the time.

Spiro also contends that Unsworth made himself a "public figure" with his desire to "milk the media coverage over what he instigated to reap a financial windfall from Mr Musk, despite the absence of any injury".

Under federal libel laws, public figures must prove that defamatory statements were made with knowledge that they were false, which Unsworth cannot do, Spiro wrote.

Unsworth's claims also are barred because Musk retracted his statement and apologised, Spiro wrote.

The response shows that Musk isn't taking responsibility for his actions and instead is blaming others, L. Lin Wood, Unsworth's lawyer, said in an email on Tuesday.

"Fortunately, Mr Unsworth does not have to tolerate Musk's antics and can ask a jury of his peers to teach him a lesson," Wood said.

Unsworth is seeking more than $US75,000 in damages from Musk, a multibillionaire. The suit also seeks a court order prohibiting Musk from making any further disparaging comments.

This is the second time in less than a year that Musk's free-wheeling comments on Twitter have saddled him with legal headaches.

Last year, Musk and Tesla reached a $US40 million settlement on allegations that he misled investors with a tweet declaring he had secured financing for a buyout of the electric car maker.

He then had to go to court earlier this year to defend himself against assertions that he had violated an agreement with the Securities and Exchange Commission about his tweeting.

Australian Associated Press