Steven McIntyre, 19, was found guilty of murdering Jack Doyle on January 3.
The High Court in Glasgow heard the single stab wound went through Jack's liver and severed two blood vessels and he bled to death.
The court was told Jack, from Greenock, had taken a knife with him to a party in Antigua Street, Greenock.
It was alleged during the trial that Jack left the flat and shouted up, challenging the occupants to a fight.
Seconds later he was assaulted by Steven McIntyre's co-accused Kevin McIntyre, 28, who was also originally charged with Jack's murder. Kevin McIntyre slashed Jack twice on the right side of his face. Steven McIntyre then confronted Jack and stabbed him once.
Kevin McIntyre pled guilty to a much reduced charge of assaulting him. Both men will be sentenced later this month once background reports are available.
This is the second time tragedy has struck the Doyle family.
Jack's aunt Elaine Doyle was just 16 when she was murdered on June 1, 1986. Her body was discovered in a lane yards from her home in Ardgowan Street, Greenock. She had been strangled.
Her killer has never been brought to justice, although her death has been subject to a cold case review.