After murdering Mr Sorrenson in his Mamaku home, Ferguson buried him in a shallow grave in a forest south of Putaruru.
During his trial he said the incident brought back a past memory of sexual abuse, and caused him to lose control.
Ferguson's appeals of his conviction, based on the argument he was not mentally fit to stand trial, reached the Supreme Court, however, all appeals failed.
Yesterday human rights lawyer Tony Ellis told Justices Terence Arnold, Lynton Stevens and Christine French, Ferguson should have been sentenced to a care facility, not prison.
When the crime was committed and the trial and sentencing took place, Ferguson was 19 years old with an IQ of
57 and very few convictions, Mr Ellis said.
"His intellectual state was not taken into account at the time of sentence,'' he said.
"A humane sentence for someone with an intellectual disability of this level would be [to be] locked up in secure care.''
Mr Ellis said the public would still be safe from Ferguson if he was moved to a care facility, and he would have the benefit of effective treatment at the facility.
"The man's already done 10 years in prison, and now can he have some movement so one day he could have a life outside of prison?''
Annabel Markham, appearing for the Crown, said Ferguson has been wait-listed for a treatment programme next
year, and the Corrections Department could allow him to start the course if he was willing to participate.
"So it's not the case that Corrections has done nothing,'' she said.
There had been numerous rehabilitative efforts made towards Ferguson, but he had been resistant to those programmes, Ms Markham said.
The order to have people serve their time in a care facility was reserved for more minor offending, she said.
"The care order in lieu of a sentence is an unrealistic option in this case.''
Ferguson failed in a parole bid this year, and would next be eligible in May 2013.
The Justices reserved their decision.
In 2010 Ferguson lost his Supreme Court bid to have his conviction quashed on the grounds he was unfit to stand trial.
After failing in a bid to the Court of Appeal, where his lawyers had argued he was not mentally fit to stand trial, Ferguson went to the Supreme Court. But the court, in a decision released in August 2010 backed the Court of Appeal's decision.