Sage, who starred alongside Stallone in Rocky 5, was found dead in his Los Angeles home Friday afternoon.
"Sylvester Stallone is devastated and grief-stricken over the sudden loss of his son Sage Stallone," Sylvester's rep said in a statement to E! News. "His compassion and thoughts are with Sage's mother, Sasha [Czack]. Sage was a very talented and wonderful young man, his loss will be felt forever."
Police, who responded to a welfare check at the home around 2:15 p.m., said there were no signs of forced entry at the home, nor any apparent foul play, and that there did not appear to be any criminal activity involved.
Details regarding the filmmaker's death are still unclear, but Sage's attorney George Braunstein told People his housekeeper first found him dead.
"We are in the initial stages of our investigation. We have no information to report at this time," Chief Coroner Craig Harvey said.