Ken Dickie, 64, died in hospital on 14 May, six days after the incident at the Rockley Park site in Poole.
Jamie Mitchell, 35, of Apple Close, had admitted intervening in an argument between Mr Dickie and his mother.
He shouted "yes" and was visibly shaking after the verdict was announced by a jury at Winchester Crown Court.
The trial heard Mrs Mitchell and Mr Dickie had been in a relationship for several years and were going through a "down phase" when Mr Mitchell visited his mother at the park on 7 May.
In the evening, Mr Mitchell accompanied his mother and Mr Dickie to the holiday park bar and got "tanked up" on alcohol.
The two men had a confrontation outside and were separated by members of the public.
Mr Mitchell had pleaded guilty to common assault in relation to the fight.
He and his mother Samantha, 57, also of Apple Close, also admitted attempting to pervert the course of justice by lying about the course of events.
The court heard the pair had initially told police that Mr Dickie had pushed Mrs Mitchell and she had pushed him back.
But Mr Mitchell later told police that he had tried to push the couple apart, causing what he described as a "tragic accident".
The pair will be sentenced on 6 February.

The wicketkeeper-batter, 26, has been a key part of the England set-up since making her international debut in 2006.
She is England's third highest one-day international run-scorer and second only to Charlotte Edwards in T20 games.
In October last year, Sussex's Taylor, who took a four-month sabbatical in 2010, became the first woman to play in Australian first-grade cricket.
The England and Wales Cricket Board said Taylor could still feature in the one-day and Twenty20 series against Pakistan beginning on 20 June.
An ECB spokesperson said they were continuing to monitor the situation and that the board would "support her, keep in touch and talk with her prior to selection for the Pakistan series and the start of the Super League".
Taylor has scored 5,581 international runs in 190 appearances, claiming 114 catches and 88 stumpings, and is one of only seven women to play in 100 ODIs for England.
Last November, her Sussex team-mate Holly Colvin retired from international cricket to take up a role with the ICC in Dubai.
And last week, England captain and record run-scorer Charlotte Edwards announced her international retirement after being told coach Mark Robinson wanted to build a new team.
A man has been found not guilty of the manslaughter of his mother's partner who died after falling and hitting his head on the ground at a holiday park.
[NEXT_CONCEPT]
England's Sarah Taylor is taking an indefinite break from cricket for personal reasons.