In a misguided attempt to help his dad, Dylan Thomas Allen-Barnes ran towards a road-side fight with a baseball bat.
When the motorist fighting the father saw Allen-Barnes fast approaching, he turned to get away but slipped and fell.
By the time Allen-Barnes arrived, the motorist was crouched on the ground covering his head.
In what was described as a callous and violent road rage attack, Allen-Barnes then hit the motorist about three times with the aluminium bat.
Allen-Barnes and his father Mark Andrew Barnes were both travelling in a truck north of Brisbane in May 2022 when they had a near miss with the motorist's car.
The motorist saw the truck pull over and thought they wanted to exchange details so he also parked off a motorway, Brisbane District Court heard.
But when he walked towards the truck Barnes got out and said: "Do you want to have a go?"
The motorist declined but Barnes punched him in the mouth and they exchanged blows.
Then Allen-Barnes got out with the baseball bat.
The motorist was taken to hospital with multiple hand fractures requiring surgery.
Two months after the attack the motorist told police that he had not been able to work in the same capacity and lost significant hand movement and functionality.
"This incident would have been both a frightening and shocking experience for him," crown prosecutor Kate Droney said.
"It's resulted in significant injuries to a member of the community who ... stopped to do the right thing."
A defence barrister said Allen-Barnes intervened with the bat because he had seen his father seriously injured in a previous altercation.
"It seemed to him that his father was not really getting the best of the situation and he's got that bat and has joined in - that's his perception," he said.
"But striking him when he's on the ground is utterly unacceptable, he accepts that completely."
He said Allen-Barnes had a relatively prejudicial upbringing and had been introduced to drugs by his mother's then partner after his parents split.
Allen-Barnes also struggled with alcohol but has taken steps to rehabilitation.
He has been prescribed medicinal cannabis for anxiety and depression, and is on a waitlist to see a psychiatrist, the court heard.
His father Barnes' defence barrister said her client had anger management problems but was seeing a psychologist.
She said Barnes had a difficult upbringing, suffering significant domestic violence before leaving home at 12 and turning to drugs.
Allen-Barnes, 24, pleaded guilty to grievous bodily harm and was sentenced to two-and-a-half years in jail, to be released on parole in December.
"I accept that your conduct involved a misguided attempt by you to support your father," Judge Joshua Trevino said.
"But bringing that baseball bat and using it on a man who by that stage was on the ground and defenceless is not in any way justified."
Barnes, 43, pleaded guilty to common assault and was sentenced to 12 months' probation.
