Police say all evidence points to 11-year-old Mike Beckenridge and his stepfather being in the car when it went over a cliff in the Catlins, despite no human remains being found.
Mike Zhao-Beckenridge disappeared almost two months ago with his step-father John Beckenridge, in breach of a custody order
Their wrecked blue Volkswagen Touareg car was found at the bottom of a cliff near Curio Bay on 6 May.
Police said all evidence collected in the investigation so far pointed to the pair being in the car when it went over the cliff.
But Southland Area Commander Kelvin Lloyd said even if they had been in the car, there was a chance nothing would be located because of the condition of the vehicle, and the fact it had been in the sea for the past two months.
He said police had sat down with family and talked through the results of the investigation.
"Understandably, they are distraught since they have not been able to find closure regarding this part of the investigation, but we are continuing to provide them with the appropriate support."
Police were still receiving information from across the country, but there had been no confirmed sightings of the pair.
The case has been referred to the coroner.