Jamie Mitchell, 35, of Apple Close, Poole, is on trial for manslaughter over the death of Kenneth Dickie, 64, last year.
Winchester Crown Court heard Mr Dickie died in hospital six days after being found with a serious head injury at Rockley Park in Poole in May.
Jurors were told Mr Dickie's death was "utterly unnecessary".
Mr Mitchell has pleaded guilty to common assault and intent to pervert the course of justice but denies manslaughter.
His mother, Samantha Mitchell, 58, of Apple Close, Poole, also appeared in court and changed her plea to guilty of perverting the course of justice.
She will be sentenced at a later date.
Prosecuting, Charles Gabb told the jury Ms 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.
Minutes later, a witness found Mr Dickie lying on the ground with a head injury.
Mr Gabb said Ms Mitchell told witnesses she was the one who had pushed Mr Dickie.
But the court heard that after his third interview, Mr Mitchell told police he had seen his mother and Mr Dickie arguing and had tried to push them apart, causing Mr Dickie to fall to the ground and hit his head on the pavement.
Mr Mitchell said it was a "tragic accident", the court heard.
Mr Dickie died in hospital on 14 May.
The trial continues.
A man charged with killing his mother's boyfriend at a holiday park called it a tragic accident, a court has heard.