Troy Kevin Taylor has pleaded not guilty to abusing and murdering baby Ihaka Paora Stokes on July 3, 2015.
The court was shown a police interview in which the 22-year-old Taylor said he had rocked Ihaka to sleep and cuddled him in the cot because the child was restless.
Ihaka was pronounced dead at 11pm due to bruising and swelling of the brain.
Taylor broke down in the police footage where he said: "I considered him my son."
Detective Nigel Thompson accompanied the defendant to the police station and said Taylor cried when he found out his partner's baby had died in hospital.
He said Taylor hung his head between his legs and shed tears.
Taylor claims the mother, not he, is responsible for the baby's death.
The trial continues in the High Court.