McKerchar, from the Hibiscus Coast, said his son was shaken up following the incident and had bruises on his chest and leg but was otherwise fine.
Cutting in front of other surfers who are already riding the wave is a fast way of provoking anger in surfers, especially locals.
Simon Egglington from Tutukaka Surf said most surfers are accommodating to learners but when some people retaliate they "take it a bit far".
"I don't know the story, the kid could have done it a few times and that's what kicked it off, that's the only way I could imagine this would happen," he said.
"If your learning or you're an intermediate surfer make sure that you're always paddling to the back of someone who's catching a wave.
"If you want to catch every wave go and sit away from everyone else, especially Te Arai because when it's on, everyone from Auckland goes there."
McKerchar said his son had avoided the surfer who attacked him all day and was surfing further down the beach than them.
However, he was unsure if there had been similar drop-in incidents in the water beforehand but said regardless, it was the wrong way to deal with it.
"He dropped in on him for sure, technically he had broken the rules and he'd been staying out of these guys' way all day," he said.
"The thing that wound me up more than anything was that it was dangerous … it seems weird a kid would have kicked a kid."
He didn't approach the surfer who hit his son but said if he saw him again he wouldn't have a go at him, instead he'd talk to him and let him know it was "inappropriate" behaviour.
