The distraught father of a brain-damaged boy prays for the man who destroyed his son's life, a judge in the Invercargill District Court said.

Aaron Kenneth Charles Howell, 24, was yesterday jailed for two years and nine months after admitting causing grievous bodily harm with reckless disregard for a 2-year-old boy's safety at Otahuti on May 29, 2009.

At sentencing yesterday, Judge Kevin Phillips said Howell had admitted his guilt only when he saw the "writing on the wall" during the fifth day of his retrial in February.

Howell had originally been found guilty of a more serious charge of causing grievous bodily harm with intent to cause the harm to the boy.

He was then sentenced to six years and nine months' jail but successfully appealed the conviction, which led to February's retrial.

Yesterday, Judge Phillips said Howell was at home with his partner and her 2-year-old son when the boy was playing in front of the television set.

Howell told him off and took him to the bedroom, with the boy happy, healthy and alert at that time.

When he came out of the room a short time later, his life had been destroyed, the judge said.

"You are the only one that knows what happened, and you do know," the judge told Howell.

"I think you lost your temper with this little boy and smashed his head into the floor," said the judge, who presided over Howell's trial.

The boy would suffer lifelong disabilities as a result of the assault.

He had up to 20 seizures a day, is paralysed down the right side of his body, couldn't walk and used a wheelchair, suffered brain damage and had to be cared for "24/7", the judge said.

The boy's paternal family had invested all their available time and finances towards the boy's wellbeing since the assault.

"The boy's father says he prays for you because he sees you as a person who assaults little children and destroys their lives.

"He says you gave his son a life sentence," the judge told Howell.

He did not believe Howell was remorseful.