The investigation into Nathan's death had stalled, but police reopened it, Mr Hercock said.
"It's not unusual for investigations to be reopened or reviewed, either after receiving additional information or further advice and that's covered by the Solicitor General's guidelines.
"In this case we have and on review the decision has been made to proceed with charges."
Police had been in close contact with Nathan's family during the investigation, he said.
"We're pleased that we can put this matter to court and give some closure to Nathan's family."
At the time of his death a Ministry of Health spokeswoman said Nathan was in the care of Idea Services, an IHC subsidiary.