"Sometimes his head goes back and forth but it's not too bad. He doesn't carry on or anything," Jesse Thomas Harvey told police during a recorded interview on April 20, 2017.

The then-19-year-old said he had been bouncing his son Casey, by holding him under his arms and lifting him up and down, for three to four weeks.

Harvey told police he spun Casey around, causing his head to go back and forth and back a second time when he caught it.

He said this occurred two days before the baby was admitted to Ballarat Base Hospital on April 19, 2017 with serious head injuries and transferred to the Royal Children's Hospital in a critical condition.

"I have spun him around before and bounced him heaps," Harvey told police.

The father's comments were part of a police recorded interview played to a jury at the County Court in Ballarat on Thursday.

Harvey, 21, has pleaded not guilty to recklessly causing serious injury and negligently causing serious injury to his seven-week-old son.

The baby became unwell and was taken to Ballarat Base Hospital five days after Harvey moved from South Australia to Sebastopol with his mother and brother on April 14.

The jury was told on Thursday Casey was severely disabled, required high-level care and had a poor prognosis.

During the police recorded interview played to the jury, Harvey said there was an incident at his Sebastopol home when he was holding Casey and sat down on a couch too hard, causing the baby's head to bump the arm.

He said Casey cried for 10 to 15 seconds, he gave him his bottle and he was fine. Harvey did not think to check for marks, saying he did not think much of it.

Casey became unwell on April 19 when he woke for a feed at 2.30am but when Harvey returned with a bottle, the baby was asleep.

Harvey told police during the interview he woke up at 10.30am and was frantic because the baby had not woken for his usual 6.30am feed.

"I rolled over and looked at him and he was staring up into space with his hands up like this," Harvey said.

"I screamed out to Mum, 'Casey's not right'."

Harvey used a taxi service to take Casey to the Ballarat Base Hospital and they arrived about 1pm, the jury had been told.

Police detectives asked Harvey if doctors questioned how Casey became injured to which he replied, "she (the doctor) asked me a few questions like how did it happen. I said 'I don't know' ".

"I would never purposely hurt him."

Detectives asked Harvey if it was possible bouncing or spinning a baby could cause head injuries. "It's possible. I can't say," Harvey replied.

When asked if he intentionally caused injury to Casey, he said, "No, not at all".

Harvey told police he had become frustrated at his son but he never shook him and described himself as a person with "pretty good patience" and he did not "really have a temper".

The trial will continue before Judge Gavan Meredith on Friday.