Mr Keno said doctors established that Leith was suffering from a bleed on his brain.
"Unfortunately his condition worsened and he died early on Thursday evening after life support was removed."
Mr Keno said a post-mortem examination has been conducted and police were awaiting the results of testing to confirm the injury to his brain led to his death.
He said police enquiries were focused on establishing the exact cause, and timing, of the brain injury.
Mr Keno said investigators were considering all options as to how Leith sustained his injuries, including the possibility that he may have been the victim of some form of assault.
"We are speaking to family members and anyone who had been in Leith's company over the last few weeks of his life in an effort to piece together what has happened.
"This is a tragic loss of such a young life and my deepest sympathies go out to all his family."
Anyone with information relevant to the investigation was asked to contact Greymouth Police 03 768 1600.
Alternatively information could be provided through Crimestoppers on 0800 555 111.