Sylvester Stallone Calls In Famed P.I. Scott Ross to Investigate Sage's Death

Sylvester Stallone has called in a private investigator to figure out how his son Sage died and what lead up to the tragedy ... TMZ has learned.

We've learned famed P.I. Scott Ross -- who was the lead investigator in the Michael Jackson molestation case and the Chris Brown/Rihanna battery case -- met Tuesday at Stallone's home with Sly, his brother Frank, and an official from the L.A. County Coroner's Office. Sources tell us they discussed Sage Stallone's death and the status of the investigation.

We're told Sly wants Ross to figure out what happened to his son -- and he's not ruling out foul play, even though he has no evidence of that. Sly also wants to know about Sage's final weeks ... whether he was using drugs or consuming alcohol. The actor also wants to know what, if anything, was troubling his son.

Ross has a long history working for the Stallone family ... we're told Sly wants someone he knows and trusts to provide him with answers about his son's death.