"He says things I think a lot of people would like to say but probably wouldn't say."
Tristan was due to release a song and a music video, which Mr Barker said was a tongue-and-cheek song called I'm a Dickhead.
"It's just a bit of fun," Mr Barker said of the music video.
"He's sort of talking about not being taken too seriously," he said.
In one of his posts, the 18-year-old called the Colorado theatre shooting last year, in which 12 people died, a "social experiment".
"I don't give a f*** about the victims, I don't give a f*** about their families, I don't care, no one should care," he wrote on Facebook. He went on to request Americans "end themselves".
"I wanted to see if you could create disgust," he said this week.
Barker gained notoriety early last year after trolling the Facebook page of pop star Nicki Minaj with an abusive rant.
Melbourne Detective Senior Constable Rodney Andrew said Barker's absence had stalled the inquiry.
Read more: Son of former Split Enz drummer wanted by Oz police
- APN