The “Rocky” actor’s eldest son tragically died in 2012 at the age 36 due to coronary artery disease.
Sage appeared as character Rocky Balboa’s son Robert in 1990’s “Rocky V” alongside his father, 77.
In the sports flick, Robert felt cast aside as his father teaches boxer Tommy Morrison how to fight in the ring.
Stallone admitted in his documentary how this plot line reminded him of his own relationship with his late son.
“Unfortunately, yes,” he sighed when asked if the story idea drew inspiration from his life.
“I try to take something that actually is what I wish I had done in real life, but I wasn’t able to do that in reality. And so quite often I would do it theatrically, magically… A lot of that is true,” Stallone added. “Unfortunately, you put things before your family. And the repercussions are quite radical and devastating.”
However, by the end of the movie, the father and son bond again over their love of boxing.
Sage appeared with his father in 1990’s “Rocky V.”©United Artists/courtesy Everett / Everett Collection
In addition to the present-day footage, the documentary plays footage from a “Rocky V” premiere. Standing with Sage on the red carpet, Stallone says, “There’s a line in the movie, ‘I’m so glad he’s born because now I can live through your eyes.’ That, I think, is what fathers look to their children for. It’s an extension of, a slice of immortality. As long as he’s alive, your memory will always be alive, that you did something right. I mean, you hope for that.”
Sage was found deceased at the time in his apartment, with his cause of death first being cited as an overdose of pills.
But the cause was later determined to be from atherosclerosis, which is the formation of fats and plaque on the artery walls and is one of the main causes of coronary-artery disease.
Sage died at the age of 36 in 2012.WireImage
Sage was then shown to have no drugs present in his system, except for a small trace of Vicodin.
Shortly after his son’s passing, Stallone opened up in a teary interview with NBC’s “Today.”
The “Expendables” star said that “time will hopefully heal” the sadness he feels over the death of Sage.
He died due to heart problems and was found dead in his apartment. Ron Galella Collection via Getty Images
“It’s tough, I mean, it’s very, very tough,” Stallone went on.
“But if you have good friends and your family’s support, it’s just something that’s a horrible situation. But, you know, time hopefully will heal and you try to get through it. But it’s just something that’s a reality of life.”
“It’s important to get back and try to start re-living your life,” Stallone continued. “Otherwise you can, you can go into a spiral.”
The action star is also a father to daughters Sophia, Sistine and Scarlet with wife Jennifer Flavin.
He shared Sage with ex Sasha Czack, and they also had son Seargeoh, 44.