The mother of a teenage boy who was killed during a sleepover says she is grateful justice has been served for her son after nearly three decades.

Arthur Haines was sleeping in a house in Waterloo, in Sydney’s inner south, with other children on April 9, 1998, when the house went up in flames.

The 13-year-old suffered severe burns when he became trapped on the top floor of the home and died in hospital 11 weeks later.

Arthur Haines, 13, was staying at a friend’s house in Sydney’s Waterloo when a fire ripped through the home. (Supplied)

Police later determined the house fire was lit deliberately.

After more than 27 years, Gregory John Walker admitted unlawfully killing the schoolboy on Wednesday.

“I’m guilty, Your Honour,” he told the NSW Supreme Court.

Arthur suffered severe burns and died weeks later. (9News)

Arthur’s mother Julie Szabo said it was the news she had been waiting decades to hear after the death of her cheerful “mummy’s boy”.

“All I ever wanted was the truth and he’s finally admitted it,” she said outside court.

“I’m so grateful justice has been served.”

Gregory John Walker pleaded guilty to manslaughter. (Supplied)

Szabo thanked police for never giving up on the case and leaving no stone unturned in the nearly three decades since her son’s death.

“This is the biggest day of my life,” she said.

“We’ve constantly waited for this day and we thought it would never ever happen.”

She’ll be marking the occasion by visiting a tree she planted in her son’s honour and decorating it with flowers.

Julie Szabo, the mother of Arthur Haines. (9News)

Police in 2020 increased the reward for information in the cold case from $100,000 to $1 million, hoping it could finally lead to a conviction.

Walker was arrested in 2022 and charged with murder and two maliciously wounding offences.

But, after his trial was postponed the day it was to start, he pleaded guilty to the lesser charge of manslaughter on Wednesday.

A second charge of maliciously inflicting harm on another person will be taken into account on sentencing.

The 58-year-old will return to court in November for a sentencing hearing.