The 17-year-old attacked Gordon Friel, 40, in Paisley last September, leaving him with a string of injuries including bleeding on the brain.
He had earlier pled guilty to assaulting Mr Friel to his severe injury and to danger of his life.
The teenager, who cannot be named for legal reasons, was jailed for three years and four months.
He was initially charged with attempted murder for what the judge described as a "savage assault".
Two girls - aged 17 and 16 - have also admitted a charge of assaulting the tannery worker.
The 17-year-old girl was ordered to carry out 150 hours of unpaid work and the 16-year-old 100 hours. Neither of the girls can be identified.
The High Court in Glasgow heard the victim was set upon after he tried to act as a peacemaker when two teenagers began arguing.
Mr Friel had been standing in Paisley's High Street eating a takeaway when the row broke out.
He approached the pair and told the bigger of the two the smaller one did not want a fight.
Prosecutor David Taylor said: "At this point the 17-year-old girl became involved, dragging Gordon to the ground and punching and kicking him on the body.
"She was joined by the 16-year-old girl who shouted 'get him' and began punching him."
Mr Friel managed to get away before the 17-year-old boy arrived, the court was told.
The boy - who wrongly believed Mr Friel had attacked the older girl - and others then chased him to Forbes Place.
The group went on to stamp on the victim's head, leaving a "footwear mark" on his head.
He suffered bleeding in the brain, a liver injury and rib fractures, and needed three operations.
Mr Taylor said: "Before the incident Mr Friel weighed around 11 stone. On his discharge from hospital his weight was five-and-a-half stone.
"He was so weak he had to use a Zimmer frame to aid walking."
The court was told Mr Friel is now back at work and is expected to make a full recovery, although he has no memory of his ordeal.
The 17-year-old boy's lawyer, Billy Lavelle, said his client was now "ashamed of his actions".
A teenager who stamped on a man who was trying to break up a fight has been jailed.