The "Rocky" actor has been left devastated after Sage, his son from his first marriage to Sasha Czack, was found dead at his Studio City apartment, reported TMZ Online.
It is said that the 66-year-old Sylvester wants to know what caused his son's tragic passing.
Sylvester, his brother Frank and an official from the Los Angeles County Coroner's Office met with private investigator Scott Ross - who was the lead investigator in the Michael Jackson molestation case and the Chris Brown/Rihanna battery case.
According to reports, Sylvester wants Ross to figure out what happened to Sage, and even though there is no evidence of foul play, he is not ruling that out.
He also wants to know about Sage's final weeks, whether he was using drugs or consuming alcohol and if there was anything troubling his son.
An initial autopsy on Sage's body, proved inconclusive and the cause of death is not expected to be known for up to two months until the toxicology tests are completed.