One of the killers of 10-year-old Damilola Taylor will be freed next month, a spokesman for the family's charity trust has said.
Damilola's father Richard is in a "very dark place" after learning that Rickie Preddie will serve only half his eight-year sentence, Gary Trowsdale said.
In 2006 Rickie and Danny Preddie both got eight-year terms for manslaughter after Damilola's killing in 2000.
The Ministry of Justice said it would not comment on individual cases.
Killers 'not reformed'
Nigerian-born Damilola had only been in Britain for a few months when he was found bleeding from a leg wound in a stairwell on a housing estate in Peckham, south London, in November 2000.
It is thought he was attacked as he made his way home from an after-school club and was jabbed with a broken beer bottle.
There have been three trials and the brothers Danny, 22, and Rickie Preddie, 23, from Peckham, went on trial in 2006 for murder after new forensic evidence was found.
Speaking to BBC London 94.9, Damilola Taylor Trust spokesman Mr Trowsdale said: "It's a very, very dark place that Richard finds himself in today.
"His wife Gloria died two years ago and she never ever got over the death of Damilola.
"Today Richard's at home, he has turned his phone off. It is something he is really struggling to come to terms with.
"People don't seem to be given the terms that normal, decent people in society expect them to get."
He added that Mr Taylor had been receiving regular feedback from the Prison Service on the Preddie brothers, who were 13 and 12 at the time of the killing.
Mr Taylor has been told the boys have not reformed.
"These boys probably needed, for their own purpose in life, stiffer sentences," Mr Trowsdale said.
"They certainly needed to be better reformed. That's certainly the thing that is upsetting - that there's been regular feedback coming out of the Prison Service that these boys haven't reformed.
"That's a big concern, because if they are not ready yet to be going back into society, then they surely shouldn't be."
He said the trust would look towards helping Rickie Preddie rehabilitate following his release.
"The boy's going to need mentoring. We would like to think that he will go on to a useful life and put something back into society.
"That, from a common decency point, is what the Damilola Trust is about."
A statement from the Ministry of Justice said: "If an offender is given a determinate sentence from the courts they are automatically released at the half way point of sentence and then subject to strict supervision in the community for the remainder of their sentence."