Zev "Velvel" Serebryanski, 61, denied the three charges of indecent assault and one count of sexual penetration of a child aged between 10 and 16.
But a Victorian County Court jury on Friday afternoon returned guilty verdicts, finding he did abuse Mr Waks on one night sometime between June 1986 and June 1988.
Mr Waks, an advocate for victims of child sexual abuse in Jewish communities, has given AAP permission to identify him as the victim.
He said he had now held both his abusers - Serebryanski and convicted rapist David Cyprys - to account.
"While there has been a significant cost in pursuing justice over the decades, emotional, mental, financial, familial and in other ways, I feel grateful and blessed to be in this unique position," Mr Waks said in a statement on Friday.
"It's been well worth it, both personally and for our community."
Mr Waks encouraged others with any information on allegations of child sexual abuse to report them to the police.
"My message to victims and survivors is to please know that you're not alone. Please reach out to your support networks and look after yourselves," he said.
Prosecutor Jason Harkess told the jury Serebryanski was in his early-20s when he developed a sexual interest in Mr Waks, who was aged between 10 and 12.
They were both part of the ultra-Orthodox Jewish community in Melbourne's southeast and they regularly attended the Yeshivah Centre.
It was at the centre on one night during the religious Shavuot festival that Serebryanski assaulted Mr Waks.
Dr Harkess told the jury Mr Waks went upstairs, away from the other men and boys who were attending the festival, and Serebryanski followed.
Serebryanski sat down on a bench next to Mr Waks and groped at his groin before pulling him into a bathroom and sexually assaulting him.
The jury was told Mr Waks blacked out during the assault and was in a state of shock and confusion when he went back home.
Mr Waks reported the abuse to police in 1996 and he confronted Serebryanski about the allegations in February 2017.
In a recorded conversation, Serebryanski told Mr Waks he never wanted to harm him, he loved him and he had only wanted to do what Mr Waks wanted.
Serebryanski's barrister Ian Hill KC told the jury his client denied any of the offending took place.
He argued Serebryanski explicitly denied in the recorded conversation that anything happened during Shavuot at the Yeshivah Centre.
Mr Hill also raised concerns over the police investigation and the fallibility of memory, given decades had passed.
The jurors retired to consider their verdict on Tuesday and returned their guilty findings on Friday afternoon.
Serebryanski will face a pre-sentence hearing in the County Court next Friday.
His bail was extended.
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
