"He was a different boy". The family of one of George Pell's victims share their heartbreak.

Warning: This post features details of child sexual abuse that could be triggering for some readers.

The family of one of the boys sexually abused by George Pell have spoken about how their son’s life deteriorated after he was abused.

The boy was one of two choirboys, aged 13 at the time, abused by Pell in the priests’ sacristy at Melbourne’s St Patrick’s Cathedral in 1996.

His family spoke to journalist Louise Milligan for her book Cardinal: The Rise and Fall of George Pell and a Four Corners investigation, which will air on Monday night.

Mamamia’s daily news podcast The Quicky explains the George Pell case.

After the abuse occurred, the boy’s family watched his life spiral out of control.

“He went from being this lovely boy, who used to come to the football with me, who used to go and help his grandparents and helped around the house, to this boy wanting to go out all the time,” his father said.

“His schoolwork, I noticed that it started slipping. His whole attitude changed. His whole being just, he was a different boy.”

A year later, when he was just 14, the boy got into drugs – including heroin.

His habit continued for 15 years. He died in 2014, at 30 years old.

His mother had asked him if he had been a victim of abuse more than once before his death. It was not until a year later, when the other victim made a formal complaint, that she learned what had happened to her son.

When his mother found out about the abuse, she told Milligan she was angry. Angry at her son for not telling her, and angry at the Catholic church.

“I sent my child there – I sent both of my children there – for an education, to be safe. You send your kids to school to be safe. Not to have this done.”

His father said he had mixed emotions about Pell’s trial and conviction.

“I’m glad that it’s over and it gives me an idea of why my son went through hell. Why he did the things he did,” he said.

“And myself, I’m just disgusted. I’m disgusted in the Catholic Church. It’s let people down. It let my son down.”

George Pell was convicted of five counts of child sexual abuse in December. He maintains his innocence and has appealed his conviction.

Louise Milligan’s investigation, Guilty, will air tonight on Four Corners at 8.30pm on ABC TV and iview.

If this post brings up any issues for you, you can contact Bravehearts (an organisation providing support to victims of child abuse) here.

If you are concerned about the welfare of a child you can get advice from the Child Abuse Protection Hotline by calling 1800 688 009, or visiting their website. You can also call the 24- hour Child Abuse Report Line (131 478).