He said the injuries were the worst he had seen in his 30-year police career, comparing them to injuries suffered by murdered 6-year-old Hastings boy James Whakaruru in 1999.
On Monday, Foster told Hawke's Bay Today that police were awaiting the medical reports, before any decisions about charges and arrests were made.
However, he said, the boy was recovering well.
"I understand he will always have significant issues but is doing extremely well and is happy."
In July this year, retired Napier Detective Sergeant Keith Price called for the removal of the right to silence for those involved in abuse against children.
Price, who spent 28 years in the police, said he had been looking to change the law since the killing of 3-year-old Moko Sayviah Rangitoheriri in Taupō in 2015.
"There'd be a lot of work to refine this, but I feel it'd be a great outcome to protect young children and remove the right of silence to these ruthless people," Price said. "It's so difficult when carrying out an investigation like this when witnesses and offenders hide behind the right to silence."
On Monday, Oranga Tamariki's deputy chief executive children and families south, Alison McDonald, said the agency could not comment in detail about the boy's situation because of ongoing police investigation and family court matters.
"Supporting this young boy's recovery is really important and we are continuing to work alongside his extended family and professionals with this in mind."
On the day of the family harm incident his stepmother and father, along with some of his extended whānau, were at the property.
The boy was accompanied by a family member when he was taken by ambulance, initially to Hawke's Bay Hospital, and then to Starship Children's Hospital in Auckland.
He was treated there before regaining consciousness in April and being transferred to a rehab centre.
