Alastair Irvine, who was in America to receive treatment for a drug problem, had his 16-month sentence cut by working six hours a day as a prison caretaker.
Lieutenant Paul Sanchez, of Pleasant Valley State Prison in California, said: "Mr Irvine's behaviour from what I can tell has been exceptional here."
Irvine, 26, was jailed in October of last year for terrorising 19-year-old Karel Taska, whose cheerleader girlfriend, Nicole Healy, he had become obsessed with.
'No problem'
The prison expects Irvine to be released after he has served just over half of his sentence.
"
He basically works as a janitor sweeping floors and generally keeping the unit tidy
"
Pleasant Valley State Prison
"There have been no problems, he's been keeping himself to himself, doing his job and if he continues this behaviour he will definitely be out sometime around March 30," Mr Sanchez said.
Irvine, who is being held in the "sensitive needs" wing to protect him from other inmates, also spent four months on remand in the state's Orange County Jail before being sentenced.
Once he is released he will be deported to the UK and never allowed to return to the US.
Describing Irvine's work Mr Sanchez said: "He's currently assigned to a job as a porter in the housing unit.
"He basically works as a janitor sweeping floors and generally keeping the unit tidy."
Family connections
The decision to hold Irvine in a wing reserved for paedophiles, convicted police officers and other "at risk" prisoners was taken because of his family connections.
Irvine was placed in the unit after his lawyer said he could be targeted if other prisoners discovered his father was the head of the judiciary in England and Wales and a close friend of Tony Blair.
Mr Sanchez said: "He was placed there for his own safety.
"Sometimes it's better to play it safe."
Paint stripper
Irvine was jailed after pleading guilty to six charges, including stalking and carrying a concealed weapon without a permit.
Mr Taska and Miss Healy are said to have been so terrified by Irvine that they moved home and gave up their college courses.
During the case, at Superior Court in Newport Beach, California, it was claimed that Irvine began stalking Mr Taska after he took an interest in his girlfriend.
Irvine threw paint stripper on Mr Taska's truck and went into the tanning salon where Ms Healy worked with a gun in his waistband, threatening to hurt Mr Taska.
Irvine is not thought to have received any visitors during his prison sentence, although the Lord Chancellor is said to be standing by his son.
His lawyer said Irvine's drug problems were behind him.