Baby Cezar rolled his eyes in his head and "wasn't the same'' after his mother's boyfriend slapped him in the back of the head three times and dropped him in his cot, a court has heard.
James Hemana is on trial in the High Court at Auckland, accused of murdering Cezar by violently shaking the six-month-old in July last year.
Hemana has pleaded not guilty to murder but has admitted to failing to provide the necessaries of life.
The Crown says Hemana also prevented the baby's mother, Victoria Taylor, from taking Cezar to hospital.
Ms Taylor was in a relationship with Hemana for about three months last year.