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.

Hanson UK had applied to use Sutton village to prevent mixer lorries getting stuck in A40 roadworks.
A county council report said lorries bound for the new Westgate Centre had been seen using the route before permission had been given.
The firm has apologised for the breaches.
In September 2015 the company was given planning permission by Oxfordshire County Council for a mobile concrete plant at Dix Pit in Stanton, on condition lorries used the A415 and A40 and avoided Sutton.
In November, Hanson said lorries bound for the new Westgate Centre were spending so much time in roadworks that 100 tonnes of concrete, worth more than Â£22,000, had been lost through prematurely setting while still in mixer lorries.
It applied for a change in the planning permission conditions to allow it to use the B4449 through Sutton.
However, council officers reported lorries were using the route before the request was approved, with a driver stopping to ask a council officer directions on one occasion.
The authority said it was "extremely disappointing" Hanson UK had not stuck to the agreement.
The council report said Hanson had apologised for the breaches and insisted it had warned drivers not to use the route.
County councillor for Eynsham, Charles Matthews, said the measures taken by the company "clearly hadn't worked".
"The trouble is the roads are not brilliant because its very narrow, the verges have been driven over and drain covers are sunk into the road," he added
If the council approves the request, 10 loaded mixing trucks could use the route each day until November 2016.
A man charged with killing his mother's boyfriend at a holiday park called it a tragic accident, a court has heard.
[NEXT_CONCEPT]
A contractor supplying concrete for Oxford's new shopping centre has been criticised for breaching an agreement not to drive lorries through a village.