The family of one of the boys sexually abused by George Pell have revealed their sadness and anger at watching their son's life spiral out of control in the wake of his abuse.

Key points: The father of one of George Pell's victims says his son was "a different boy" after the abuse

The father of one of George Pell's victims says his son was "a different boy" after the abuse Police reveal the Catholic Church had been "difficult" to deal with.

Police reveal the Catholic Church had been "difficult" to deal with. Family and police praise Pell's surviving victim for speaking out

The boy was one of two 13-year-old choirboys molested by Pell in the priests' sacristy at St Patrick's Cathedral in 1996.

His father has told Four Corners how they watched their son change from a cheerful young kid — with no idea why.

"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," he said.

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

Andrew La Greca was a member of the choir at the time the boys were abused and he also noticed the change in one of the victims.

"He was just a different boy. Just totally different. Just didn't care about anything. Didn't care about anyone. Just didn't even want to breathe," he said.

The St Patrick's Cathedral choir in 1996. ( Supplied )

Within a year of the abuse, the boy had started using heroin.

"I noticed some bits of foil, that had been burnt. My parents noticed that — his grandparents," his father said.

His mother twice asked him if he had been sexually abused, but he never revealed what had happened to him.

"It's devastating to watch your child spiral out like that. It was very hard to watch," she told Four Corners.

Their son died of a drug overdose in 2014.

It wasn't until a year after his death, when the other victim made a formal complaint about the abuse, did the family realise what had happened to their son.

The father says he has mixed emotions about the 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."

Lawyers for Pell say he maintains his innocence and an appeal has been lodged.

Police found Church 'difficult' to deal with

Police Sergeant Doug Smith ran Taskforce SANO when it first started investigating George Pell in 2015. ( ABC News: Jeremy Story Carter )

Doug Smith was the sergeant overseeing Taskforce SANO's investigation into Pell.

He told Four Corners that, over the years, the police tasked with investigating child sex abuse cases in the Catholic Church found the church "difficult" to deal with.

"They were openly saying that they would cooperate, but I think you could almost say that the way that they classed their cooperation would be similar to a protester lying on the ground in the middle of the street not resisting the police, but the police would have to pick that person up and drag them off the street," he said.

"I think that that's the level of cooperation that the Catholic Church gave us."

Late this afternoon the Catholic Archdiocese of Melbourne released a statement saying: "The Archdiocese of Melbourne cooperated fully with the police at all times during their [Pell] investigation."

While Pell was cooperative at all times, Sergeant Smith says they faced arrogance from him.

"There's a level of arrogance that some people get to where they're at this position in standing in the community. When you are seen to be someone with some level of importance, I think there is a level of arrogance that goes with that. And he's not going to hide that from the police," he said.

A police photograph of the priests' sacristy at St Patrick's Cathedral where the abuse took place. ( Supplied )

The family and police have praised the bravery of the victim who came forward and whose testimony ultimately convicted Pell.

The victim has asked for his identity to be kept private in the wake of the guilty verdict.

"He's a credit to himself and he needs to be congratulated because he's done a very, very good job at being believed," Sergeant Smith said.

"All you can do is tell the truth, he can't do any more than that. And for a jury of his peers to believe him, I mean, it's fantastic.

"No-one's above the law. The Catholic Church isn't above the law."

Andrew La Greca was a member of the choir at the time the abuse took place. ( ABC News: Jeremy Story Carter )

Andrew La Greca is glad justice has been served for both his fellow choirmates, and he praised the victim who spoke out.

"He's a hero. I think deep down, he wasn't just doing it for himself. Deep down he was doing it for someone else as well," he said.

Watch Louise Milligan's investigation, Guilty, tonight on Four Corners at 8.30pm on ABC TV and iview.