Willem Bowen Scheeren, now 58, befriended the parents of the child through a Cub Scouts group in Canberra in the late 1970s.

The relationship became so strong that the parents would frequently let Scheeren, then 23 and 24, care for the boy, aged 11 and 12, alone.

But Scheeren, unbeknownst to the victim's parents, had an attraction to adolescent boys, and had himself been subjected to sexual abuse, trauma and humiliation throughout his childhood.

Scheeren is the first child sex offender brought to justice under ACT reforms that closed a loophole preventing the prosecution of some historical molestation cases that were more than 12 months old.

His crimes against the boy began in his shoe shop during school holidays in 1979, when he showed the child pornography and began touching him inappropriately.

That was the first of four separate ordeals of abuse in 1979 and 1980, which would culminate in 12 criminal charges laid this year.

One of the gravest breaches of the family's trust occurred when Scheeren was at their home watching television on a couch next to the boy.

He put a blanket over himself and the boy and molested him for 45 minutes as the boy's and a family friend sat in the same room.

Scheeren was sentenced in the ACT Supreme Court on Thursday to eight years imprisonment, but he will be eligible for parole in May 2017.

Chief Justice Helen Murrell described the crimes as a "gross violation of the close relationship between the offender and the victim's parents".

"I acknowledge the terrible impact that these offences must have had on the victim," she said at the start of her sentencing.

"I appreciate they would have obviously had a devastating impact on the victim's family."

But the court heard there was no evidence Scheeren had committed other crimes against children since. In fact, his criminal record was otherwise completely clean.

He had undergone counselling and sought treatment well before he was brought to court, realising the catastrophic consequences of his crimes.

Scheeren's early life experiences – which included being sexually assaulted by a relative, abused by Christian brothers at school, and watching his father fall off his bike and suffer permanent and severe brain damage – had stunted his personality and left him believing such behaviour was normal.

Scheeren had gone to the family home of his victim to apologise a decade after his abuse, but was told he was not welcome by the father.

He was interviewed by police much later and co-operated to an "unusually high" extent with their investigations.

The court found him to be at low risk of reoffending, given his age, his treatment, and the fact there were no further offences in the intervening decades.