A Christian music producer subjected his girlfriend's son to a "Spartan-like regime" in the months leading up to his death which involved him being repeatedly hit, yelled at, refused food and forced to run until exhausted, a court has been told.

Kodi Maybir, 31, has pleaded not guilty to the murder of the boy, aged seven, who was found dead in an Oatley unit in Sydney's south in May 2013.

An autopsy revealed the child had suffered skull fractures and subsequent brain swelling.

Maybir is facing 14 charges, including multiple common assault charges.

He has pleaded guilty to six of them.

The boy's mother, Kayla James, is in jail after pleading guilty to the boy's manslaughter in June.

About four months before her son died, James and her three young children moved into Maybir's unit, which was converted into a music studio.

"Evidence will show the accused very substantially controlled [Kayla James] and introduced her to a bizarre system of controlling her family, which she went along with," crown prosecutor Christopher Maxwell QC said during his opening address.

Prosecutors also told that court that evidence will show that Maybir "held extreme beliefs concerning religion ... one which seemed to be eliminating the devil from people".

The jury will watch several home videos, including one in which Maybir tells one of the boy's younger siblings to hit him, Mr Maxwell said.

The court heard that Maybir forced the seven-year-old to use nappies because there were no bathroom facilities in the studio.

Prosecutor says Maybir forced boy to wear nappies

Mr Maxwell said: "Ms James will say at times the accused instructed that [the boy], who had previously been toilet trained, should remain in nappies" so he didn't have to take him to the building's shared toilet.

The prosecutor also described a string of alleged abusive offences witnessed by campers when the family were holidaying at a campsite in Bulli, south of Sydney, in March 2013.

He said campers, who are due to give evidence, saw the boy sitting outside the family's tent early in the morning, in 16 degree Celsius weather, sobbing quietly.

A male voice could be heard calling out from the tent "that's what happens when you shit yourself, you stay out there", Mr Maxwell said.

"Witnesses will say he had to wear his underpants on his head as punishment," Mr Maxwell said.

The court heard that there were several instances in which the boy was made to run until exhausted.

Witness will say that Maybir made the boy run back and forth over a 15-metre distance and when he fell over, Maybir punched him in the face, the court was told.

On another occasion, witnesses saw Maybir running behind the boy on a beach, hitting him with a stick when he slowed down or fell over, and yelling "come on, hurry up, keep moving", Mr Maxwell said.

The prosecutor said one witness noticed the boy was not given chicken on his sandwich or cordial, like the other children.

When asked why, Maybir said "he was going commando-style" and that it was "aimed at making him stronger, and to build him into a man".

Maybir told one man that he had shaved off the boys hair and eyebrows "because he wanted to make the deceased like Neo from the Matrix and make him like a soldier", Mr Maxwell said.

The prosecution said that such evidence pointed to the control and "Spartan-like regime that [Maybir] applies to the deceased".

The trial is expected to last six weeks.