The 36-year-old was found dead in his Los Angeles home on July 13 by his housekeeper, who was asked by Stallone's mother to check on him.
The findings will be detailed in a full autopsy report to be made public next week, Corral said.
Sylvester Stallone made a public appeal last month for an end to "speculation and questionable reporting" about his son using drugs.
"Because when a parent loses a child there is no greater pain," Stallone said. "Therefore I am imploring people to respect my wonderfully talented son's memory and feel compassion for his loving mother, Sasha, because this agonizing loss will be felt for the rest of our lives. Sage was our first child and the center of our universe and I am humbly begging for all to have my son's memory and soul left in peace."
Sage Stallone was engaged to be married and was working on various film projects at the time of his death.
His lawyer, George Braunstein, said Sage "was in good spirits, and working on all kinds of projects."
"He was planning on getting married. I am just devastated. He was an extremely wonderful, loving guy. This is a tragedy."
Sage was best known for appearing alongside his father in "Rocky V," playing son Robert Balboa.
He also appeared with his dad in 1996's film "Daylight."
An avid film connoisseur, he then moved into directing and producing.
"He was a walking film encyclopedia," Braunstein said.
The younger Stallone also co-founded Grindhouse Releasing, a company that describes itself as "dedicated to the restoration and preservation of motion pictures historically held in very low regard."
Sage was the first son of Stallone and actress Sasha Czack.