Sylvester Stallone in 1994; Sage Stallone in 2006.Photo:Jeff Kravitz/FilmMagic; Albert L. Ortega/WireImage

Jeff Kravitz/FilmMagic; Albert L. Ortega/WireImage
Sylvester Stallonereflects on triumphs and regrets in thedocumentarySly. He also broaches a difficult subject: the 2012 death of his sonSage Stallone.
The actor, 77, has rarely commented on the death of Sage, who died at 36 due to coronary artery disease atherosclerosis, which caused a heart attack.
One of the somber sequences in theThom Zimny-directed Netflix doc shows footage of Sage at 14, when he played Robert, the son ofRocky Balboa, opposite his father inRocky V.
That 1990 film’s story centered on Rocky’s difficult relationship with his kid, who feels Rocky spends far more time training with boxer Tommy (Tommy Morrison) than with his family including Adrian (Talia Shire).
When asked inSlywhether his own family dynamics informed the writing of that story — much as his real-life experiences informedRocky,Ramboand more — Stallone answers, “Unfortunately, yes.”
Sylvester Stallone and Sage Stallone.Frank Trapper/Corbis

He adds, “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. Unfortunately, you put things before your family. And the repercussions are quite radical and devastating.”
Sage, whom Stallone shared with actress Sasha Czack, was found dead in his Los Angeles home on July 13, 2012.
“Sylvester Stallone is devastated and grief-stricken over the sudden loss of his son,”said a repfor the actor at the time. “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.”
Sylvester Stallone and Sage Stallone in “Rocky V” (1990).Mgm/Ua/Kobal/Shutterstock

In 1996, Sagetold PEOPLEthat when Robert yells at Rocky in the film about not being there for him, it wasn’t entirely acting.
“When I was screaming: ‘You never spent time with me! You never spent time with my mother!’ that was true. I was looking into my father’s face and really saying that,” said Sage at the time.
He added that the real-life dynamic with Stallone shifted after art imitated life while filmingRocky V: “I got a lot of things out. We broke into tears a few times. After the film, everything changed. We’d do anything for each other.”
Never miss a story — sign up forPEOPLE’s free daily newsletterto stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.
Sylvester Stallone in “Sly”.Courtesy of Netflix

Courtesy of Netflix
After playing glimpses of those scenes featuring Sage inRocky V,Slythen shows footage from the film’s red carpet premiere in 1990. Standing with his son and talking to reporters, 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.”
Stallone is also dad todaughtersSophia, 27, Sistine, 25, and Scarlet, 21 whom he shares withJennifer Flavin, hiswifesince 1997.
Slyis now streaming on Netflix.
source: people.com