Mr Witehira said members of the public stopped to help the family after the 2pm crash, about 1km south of Palmerston in a 100km/h zone.
All but the 25-year-old man were out of the vehicle by the time emergency services arrived.
The young father had a non-life threatening head injury and was taken to Dunedin Hospital in the Otago Regional Rescue Helicopter.
His son and parents were taken to hospital by ambulance.
Mr Witehira said the driver had a sprained ankle and her husband was not injured.
The milk tanker driver was "shaken" but not injured.
It appeared the northbound vehicle crossed the centre line and hit the southbound milk tanker, he said.