"It breaks my heart ... he can't do his shirt up," his mother, Mary, has told the Nine Network's 60 Minutes program. "He can't do his shoelaces up. He can't do his tie up, and to think a boy at his age, you know ... that's embarrassing for him."

Knight, then 17, suffered lifelong brain damage in January 2013 when a gatecrasher to a party he was attending in Forestville threw a metal rod at him. It lodged in his skull.

The gatecrasher, known only as DS because he was underage at the time of the assault, was sentenced to prison for at least eight years on Friday.

Mrs Knight said Liam would have to live with the consequences of that night for the rest of his life.

"What sentence can you give someone that gives somebody a life sentence?" Mrs Knight told 60 Minutes reporter Michael Usher.

Downing Centre Local Court heard DS gatecrashed an 18th birthday party in Forestville after being denied entry to the event.

Intoxicated, he climbed on to the roof of a shed in the backyard and hurled building materials at the party-goers below.

DS pleaded guilty to recklessly inflicting grevious bodily harm but the Crown proved the more serious charge of intent.

He will remain in juvenile detention until his 21st birthday, and then will be transferred to an adult prison.

Liam, who required five hours of emergency surgery at Royal North Shore Hospital after the assault, told the court in a victim impact statement that he now felt self-conscious, and was unsure of his ability to live a normal adult life.

"People no longer see me as Liam, but as someone who needs help," he said in the statement.

The full interview can be seen Channel Nine’s 60 Minutes on Sunday night.