The assaults happened at a city centre property, Port Levy on Banks Peninsula and the Waimakariri River.
The victim, now a father in his 30s, today said Charteris' "selfish needs and desires" condemned him to a life sentence of "guilt, confusion, anger and turmoil".
"David knew from his life experiences that I was easy pickings and yet he exploited that in the most despicable way," he said in a victim impact statement read to the court.
"Your employer was the organisation that should've been available to support me."
In taking the boy on long drives to isolated areas, Charteris, who continues to deny his offending, displayed planning and premeditation, and a high level of breach of trust, Crown prosecutor Kerry White said.
His crimes had a "devastating effect" on his vulnerable victim, she said.
Charteris often shook his head in the dock while the facts of the case were read out this afternoon.
"Mr Charteris maintains his innocence," said defence counsel Anselm Williams who described him a man "otherwise of good character" who had been heavily-involved in the community. He also has health issues.
But Judge Neave said his crimes were serious.
There were a "variety of sexual acts" perpetrated by Charteris on the "unfortunate complainant", he said.
While there was no suggestion of grooming or targeting the victim, the judge accepted that Charteris took advantage of circumstances to offend against the child.
In sentencing Charteris to 14 years' imprisonment, Judge Neave told him: "You will have to face up to your offending if you are to be considered for parole, it's as simple as that."
Victim Support today admitted failing to act when it received an allegation about the offending from an anonymous complaint before the police probe.
"Victim Support acted on legal advice that anonymous allegations, without the possibility of a fair investigation, offered no grounds to act," Victim Support chief executive Kevin Tso said.
"However, the organisation should have done more to ensure the allegations could be investigated by police."
The offending happened before Charteris' was employed by Victim Support. He was not employed in a client facing role, Tso said.
He ceased work immediately once charges were laid by police.
"Since then, we have begun thoroughly reviewing our processes to prevent anything of this nature occurring again," Tso said.
"We have tightened multiple HR policies, our code of ethics and our employment contracts. We are also enhancing our processes for reporting illicit behaviour."
He added: "The victim has shown remarkable courage in pursuing this outcome, particularly in the face of significant obstacles in reporting historic crimes. He has our enormous respect.
"It's imperative that victims are at the centre of the justice process, and that we lead by example."
