The 36-year-old's body was found at his LA apartment apparently surrounded by "numerous empty pill bottles", but the results of an autopsy have been deferred pending toxicology tests.
It has now been reported that Stallone met with leading Hollywood PI Scott Ross, who led the investigations into sexual abuse allegations against Michael Jackson and Chris Brown's 2009 assault on Rihanna.
According to TMZ, the Rambo star has asked Ross for information about how Sage died, including whether he was drinking or using drugs in his final weeks.
Sage's mother Sasha has suggested that surgery to remove five teeth could have led to the actor's passing.
Stallone recently called on the media to stop the "speculation and questionable reporting" about his son's death.