Sylvester Stallone's son died from a heart attack

Sylvester Stallone's son, Sage Stallone, died from a heart attack ... and no drugs were found in his system ... TMZ has learned.

The L.A. County Coroner has determined Sage died from natural causes.  As for why he had a heart attack, TMZ was told there was evidence of heart disease.

A source connected with the Coroner tells TMZ ... it was "unusual" for a person Sage's age -- 36 -- to have such advanced coronary problems, but that's what killed him.

Sage's body was found July 13, but he may have been dead days earlier.