Douglas Herridge, 18, hit Colin Payne at the family home in Dartford, Kent, in November.
Jurors at Maidstone Crown Court heard Mr Payne intervened in a row over money between the teenager and his mother.
Mr Herridge said the 54-year-old grabbed his neck and tried to throttle him, when he struck the blow.
Home Office consultant pathologist Dr Richard Chapman said Mr Payne, a former national squash champion, died from a blow of moderate force that damaged a vessel carrying blood from the heart to the brain.
Judge Jeremy Carey, presiding, said: "The word tragedy is probably overused, but this case can truly be described as tragic.
"Colin Payne was on everybody's assessment a fine man, a distinguished amateur sportsman, a good father, and a responsible and decent member of society, and he came to an untimely death in circumstances that really were absolutely devastating."

King Arthur Pendragon argued a parking fee of £15 for the 2016 summer solstice breached his human rights.
At an initial court hearing in January, Mr Pendragon said the claim was not about money or costs, but because the fee "unfairly targeted his religion".
But Salisbury County Court ruled that introducing car parking charges was a "reasonable and legal measure".
Mr Pendragon had argued that this treatment was in direct violation of his human rights and that he has a right to worship at Stonehenge without unnecessary restriction or hindrance from English Heritage, which he claims is merely managing the site "on our behalf".
English Heritage said it introduced the parking charge to encourage more people to car share or travel by bus.
However, Mr Pendragon said he wanted to prove English Heritage was wrong to turn him away when he refused to "pay to pray".
A teenager who fatally struck his stepfather with a single punch to the head has been cleared of manslaughter, because he acted in self-defence.
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A senior druid has lost his legal challenge with English Heritage over car parking charges at Stonehenge.