Tim Sullivan told the County Court jury during closing addresses on Friday his client, Jesse Thomas Harvey, was telling police the truth when he said he did not shake his seven-week-old baby at his Sebastopol home on April 19, 2017.

He said there was ample time for Harvey's brother, Peter Scott, to cause the head injuries at 2.30am while his brother went to the kitchen to get a bottle of milk for the crying baby.

"Jesse is up, goes to kitchen, prepares bottle. In that time there is ample time for an injury to take place by his brother Peter," Mr Sullivan said.

When the defence barrister asked Mr Scott on Wednesday if he shook Casey he said, "I suggest that is a load of b******t that just came out of your mouth", and "why would I shake a baby?". 

Mr Sullivan told the jury: "The prosecutor is unable to disprove the reasonable possibility Peter committed the offence beyond reasonable doubt."

"Jesse Harvey is telling the truth in his record of interview." 

But police allege the 21-year-old seriously injured the baby sometime between 2.30am and 12.46pm because he told detectives he was with the baby the whole time.

Crown prosecutor Gary Hevey said Harvey did not tell the truth during the record of interview and he "shook his baby so hard ... that this child's life is effectively over".

Casey was diagnosed with a number of head injuries, including subdural haematomas, retinal hemorrhages, swelling on the brain, diffuse axonal injury and torn posterior neck ligaments.

He is now severely disabled, requires high-level care and has a poor prognosis.

Mr Hevey said during the record of interview, Harvey did not suggest his brother or mother caused the injuries, but said his mother was a loving, caring grandmother, while his brother was an excellent uncle.

He told the jury Harvey "freaked out" when he woke about 10.30am, well past Casey's usual feeding time, to find him "staring up into space" with his hands in the air.

But Mr Hevey said it took him until 12.46pm to take Casey to hospital in a taxi.

He said Harvey went to a shop to buy credit to phone his sister and he entered the taxi with a take-away milkshake.

"This is not consistent with someone freaking out at 10.30am," Mr Hevey said.

The prosecutor drew the jury's attention to forensic paediatrician Dr Jo Tully's evidence where she confirmed Casey's injuries were caused by shaking, not by playing ring-a-ring-a-rosy.

Mr Hevey said Dr Tully believed the baby's injuries occurred closer to the taxi trip to hospital than earlier in the morning.

She had told the jury Casey's injuries were seen in children involved in car accidents or a 10-metre fall, not normal mechanisms of play, including ring-a-ring-a-rosy.

Harvey's counsel said there was a reasonable view of the facts consistent with the innocence of the accused.

He has pleaded not guilty to recklessly and negligently causing serious injury.

The trial will continue on Monday when Judge Gavan Meredith with give his legal direction to the jury.