Kodi James Maybir was stupid, an idiot, someone who lacked even "a modicum of common sense", his barrister told the NSW Supreme Court.

But it is those deficiencies that cast doubt on the claim Mr Maybir murdered a seven-year-old boy, Grant Brady, SC, said.

Mr Maybir, a Christian pop music producer, has pleaded not guilty to murder but guilty to manslaughter.

He was living in a relationship with the boy's mother, Kayla James, in a music studio in Sydney's south at the time of the boy's death in May 2013.

James is is serving a jail sentence for manslaughter by gross criminal negligence.

Originally, Mr Maybir told police the boy had died after falling off a pogo stick. He then changed his story twice more, the court has heard.

The defence case is that the boy hit his head after Mr Maybir threw him in a play wrestling move gone wrong.

"No doubt when you sat there listening to me open [the defence case] you kept an open mind," Mr Brady said in his closing submissions on Monday.

"But you also might have thought, a little bit, 'Come on.' A fake wrestling move, where he's thrown the boy so hard he's broken the back of his skull as his head hits the ground? Surely no one's that stupid."

Mr Brady said the jury had seen his client was in fact "that much of an idiot".

He replayed in court a home video from 2010 showing Mr Maybir play wrestle with a two-year-old boy.

At one point Mr Maybir flings the child on to a couch with one hand, without looking to see where he landed.

"You have proof positive someone is that stupid, that someone is that lacking in common sense to fail to appreciate the dangerousness of doing some fake wrestling that he has seen on TV on a child," Mr Brady said.

In closing the Crown case, prosecutor Chris Maxwell, QC, said that James' testimony contradicted Mr Maybir's latest story.

James had told the court she heard two distinct noises coming from the living room about the time of the alleged assault.

One sounded "as if something was scraping against the floor".

Mr Maxwell said that was inconsistent with the events Mr Maybir described.

The Crown has argued the boy died after months of abuse, including bizarre discipline techniques, at the hands of the accused.

Mr Maybir was allegedly training the boy to become a "soldier of Christ".

The trial continues.