The 36-year-old actor died on Friday (13.07.12) after a suspected drug overdose and a representative spoke on his father's behalf.
Michelle Bega, released a statement saying: "Sylvester Stallone is devastated and grief-stricken over the sudden loss of his son Sage Stallone. His compassion and thoughts are with Sage's mother, Sasha. Sage was a very talented and wonderful young man. His loss will be felt forever."
A family friend added that Sylvester is "overwhelmed and stunned" by the sudden death of his eldest son.
A source told website RadarOnline.com: "This came completely out of the blue. Sylvester did not see this coming for one minute.
"Sage was healthy, and Sylvester thought he was doing really well, this came from complete left field.
The insider added: "Sylvester was very close to Sage and he is absolutely overwhelmed with grief by his death. He is totally stunned and still in a state of shock."
He was Sylvester's son from his first marriage to Sasha Czack.
An actor, director and producer, Sage appeared in the fifth installment of Sylvester's biggest film franchise, 'Rocky', where he played the title character's on-screen son, Robert Balboa.
Most recently, Sage appeared in a 2010 movie called 'Promises Written In Water' and a short titled 'The Agent'.
His short-lived marriage to Starlin Wright in May 2007 was annulled after just 10 months. -BANG Showbiz