"He did a jump and something went wrong and the bike landed on top of him, and that's where the burns came from.
"I think the burns are quite severe."
An off-duty emergency department nurse helped to treat the man at the scene before paramedics arrived in the Westpac Rescue Helicopter.
The man was then flown to the Wellington Hospital emergency department.
Mr Alsop praised the actions of the young boy.
"Under the circumstances the son did amazingly well.
"He was understandably upset, but kept his cool and managed to perform CPR, which is also a credit to his school for teaching him."
But he said people needed to be aware of the hazards of riding on a beach.
"People should wear safety gear when they're riding a two-wheeler on the beach, and they should be aware of the fact that the beach is a public place so they need to watch out for others."
He said that after the alarm was raised, medical gear was taken from the Riversdale Surf Club to assist the patient.
Sergeant Peter Rix of Masterton police confirmed the man had not been wearing a helmet.
"He was bleeding from the head so there was some concern," he said.
Riversdale Surf Life Saving Club captain Mike Taylor said that it was very common to have vehicles, including motorcycles, on the beach.
It is believed the injured man will have plastic surgery for his burns.
