The man, known only as BIW, told the Royal Commission into Institutional Responses to Child Sexual Abuse on Thursday that his mother was then billed $2500 in unpaid fees, and threatened with legal action if she did not pay.

It was only when the single mother engaged a lawyer to fight the matter that the school dropped its threats.

The man who abused him was later convicted of abusing more than 40 boys.

But the victim said authorities tried to sweep the matter under the carpet and behaved aggressively to him when he told them of his ordeal.

The victim started at the school's boarding house, Highton, in 1989 when he was 14.

He had only been there for two weeks when he woke in the middle of the night to find a man's hand beneath his bedclothes, masturbating his genitals.

The man told the commission he lay still in fright before stretching and then turning onto his side away from the man. Finally, the man stopped and left the room.

"I stayed laying in my bed extremely scared and not believing what had just happened to me. I couldn't sleep or do anything. I was scared he could come back at any time and do this again," he said.

The next day the boy told his mother and the school matron, Catherine Parsons, what had happened to him.

That night he was called to a meeting with the Highton master Robert Bugg and deputy master Paul Claridge.

There, he was grilled about what had happened to him.

"I remember the vice principal was quite mean to me and I felt they didn't believe me," he said.

"It was as though I had broken into these people's homes and destroyed everything they loved; it was just surreal."

Mr Claridge, in particular, was very aggressive: "He just did not want to know."

The man said he spoke to his mother later that night, and she told him the school had instructed her that he was not to speak about what had happened to him until the school had "looked into it".

The next day, however, the boy got talking to two other boys who told him they had seen a man in the halls near his room the night before. The boy told Mr Claridge about what the other students had told him, excited to have information to give the school.

Instead, he said, Mr Claridge "exploded".

The teacher dragged him to an office, he said, leaving him there alone for six hours.

Finally, the boy's mother arrived and told him he had been expelled. The pair packed his belongings and left.

"I was tormented for years knowing that I had been expelled from school for reporting the truth," the man said.

"My abuse was never reported to police. I felt terrible guilt that I had left other children at the hands of a sex offenders. I knew that other children would be abused because no one would listen to me."

In 2005, a former Geelong Grammar boarding assistant, Philipe n, was convicted of sexually abusing 40 male students between 1985 and 1995, and sentenced to 6½ years' jail.

He was later convicted of, and pleaded guilty to, abusing BIW, and received a 12 month sentence, wholly suspended.

The royal commission continues.