Justin Dennis James Martin, 30, was convicted on one count of defensive homicide and jailed for eight years.

In the Supreme Court at Warrnambool on Friday, Martin was ordered to serve five years in jail before becoming eligible for parole, but has already served 478 days.

Martin suffered a lack of oxygen for six to eight minutes at birth and was certified as intellectually disabled by the Department of Human Services in June 2001.

His father testified in court Martin had the mental capacity of a 10 to 12-year-old.

Martin murdered Alan Baker, 79, on January 27, 2010, while the pair were watching tennis at Mr Baker's Comyn Street home.

Martin alleged he king hit Mr Baker, kicked him in the head and stomach and stabbed him twice, before seeking help from neighbours.

But a post-mortem by Dr Marian Wang on January 29 revealed seven distinct stab wounds; one to the right of the neck, four to the anterior chest wall, one to the abdomen and one to the back, as well as blunt force trauma to the face and head.

Martin, who had known the victim for six months before the incident, told police he had reacted in anger after Mr Baker made sexual advances while they were watching television and having a beer on January 27.

"He tried to touch me up and I said no and then he tried to again and then things got out of hand," Martin said in a police statement.

"He sat on top of me and I got sick of it and I told him not to and the more he done it the angrier I got."

Justice Elizabeth Curtain said Martin's allegations were supported by DNA evidence and the statement of a man who went to police after he heard of Mr Baker's death to testify the deceased sexually abused him from the ages of 10 to 14.

But she said that and other factors did not excuse Martin's actions.

"I declare, that, were it not for your plea of guilty, I would have sentenced you to 10 years with a non-parole period of seven years," Justice Curtain said.