Pax was 16 when he used a private Instagram account for his rant, which is likely to ramp up the animosity between Pitt and his ex-wife Angelina Jolie.
He wrote in 2020: “Happy Father’s Day to this world-class a**hole!! You time and time and again prove yourself to be a terrible and despicable person.
“You have no consideration or empathy toward your four youngest children who tremble in fear when in your presence.
“You will never understand the damage you have done to my family because you are incapable of doing so.
“You have made the lives of those closest to me a constant hell.”
Pax, adopted by Jolie in Vietnam in 2007, accompanied the rant with a photo of Pitt collecting an Oscar.
He continued: “You may tell yourself and the world whatever you want but the truth will come to light someday.
“So Happy Father’s Day, you f***ing awful human being!!!”
An insider told the MailOnline: “It’s the account he uses for friends — friends from school mostly. He never says much about his parents. He keeps himself to himself, so that was unusual.”
Jolie, 48, told Vogue in 2021 that she had left Pitt, 59, “for the wellbeing” of her family.
She had filed for divorce in 2016, two years after they wed and just days after reports of a clash between Pitt and Pax’s older brother Maddox on a private jet.
Allegations Pitt had been drinking and Maddox stood up for his mum during a row sparked a probe by the FBI and the Los Angeles Department of Children and Family Services.
Jolie initially took full custody of their six children, but a new arrangement in 2021 allowed them to spend 50 per cent of their time with Pitt.
However the decision was revoked after it turned out that the judge on the case had close business relations with Pitt.
There is an ongoing £280million legal battle between the A-listers over a vineyard in France.
Jolie sold her half, saying she is no longer comfortable selling alcohol.
She has lashed out at her former husband, labelling him a “bully”.
Three of the couple’s children were adopted, including Maddox, now 22, and Zahara, 18.
The couple also have their biological children Shiloh, 17, and 15-year-old twins Knox and Vivienne.