Christopher Reeve's Son with Ex Calls Mom 'Incredibly Brave to Tell Her Side of the Story' in New Doc (Exclusive)

Christopher Reeve 's son Matthew Reeve is "proud" of his mother Gae Exton for opening up about her late ex in a moving new documentary.

At the New York City premiere of Super/Man: The Christopher Reeve Story on Wednesday, Sept. 18, Matthew, 44, explains why Exton, whom the late Reeve was in a relationship with in the late 1970s and '80s, took part in the film.

"Once we decided we were going to get to this, other family members came on board," Matthew says, when asked if his mother's participation required some convincing. "I mean, once we kind of said we were going to commit to this, we were going to tell the stories we hadn't told."

"We were going to share the memories we never shared before on camera. My mom is incredibly brave to tell her side of the story and I'm proud of her," Matthew adds.

Christopher, the Superman actor who was paralyzed in a 1995 equestrian incident and died in 2004 at age 52, also shared daughter Alexandra with Exton, a British modeling agent. Following their split, Reeve married actress and singer Dana Reeve and welcomed his youngest son, Will Reeve , now 32. Dana later died of lung cancer in 2006 at 44.

"Dana was really the family documentarian, she always had the camera out, she would film things really like, especially if dad couldn't be there so that he'd be able to watch, but she always used it," Matthew tells PEOPLE of home video footage seen throughout the new documentary. "We're very lucky that we had video cameras growing up."

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories.

Matthew's sister Alexandra tells PEOPLE she felt her mother Exton's participation "was powerful for her to open up," given her private nature and notes that Exton was witness to Reeve's early rise to fame in the '70s.

"My dad was a young actor who nobody knew to suddenly being on this rocket ship to fame, and she had a front row seat for all of that," Alexandra says. "So it was beautiful to hear her tell that story, and then tell us her love story of her own relationship. And then the continued the love story, even after they separated, about how our family still remains really close, because they gave us this gift of having the united set of parents, even after they separated. So I'm so glad that was celebrated."

Matthew additionally tells PEOPLE that bringing their father's story to audience members who may be too young to remember Christopher's Superman movies "means everything" to him. "It's a real honor to just be in a position to help tell his story for the best of my own memories," he says.

Super/Man: The Christopher Reeve Story, from directors Ian Bonhôte and Peter Ettedgui, is in select theaters Sept. 21 and Sept. 25.