Christopher Reeves' son Will reveals his last tragic promise to his father before the Superman star's death

Will Reeve, the youngest son of Christopher Reeve, has revealed the powerful promise he made to his father shortly before his death.

The legendary Superman actor died unexpectedly of heart failure in 2004 at the age of 52, nine years after being paralyzed by a horse riding accident.

Now the film legend's three children, Matthew, 44, Alexandra, 40, and Will, 32, have collaborated to bring his life to the big screen in the insightful new documentary “Super/Man: The Christopher Reeve Story.”

In a clip aired on Good Morning America on Tuesday, Will recalled the moment when they all had to say goodbye: “I told him I love him. I'm going to do everything I can to make him proud.”

Alexandra said of the heartbreaking farewell: “[Dana] comes flying and she just screamed, 'I love you, I love you!' over and over. She made sure he could hear. She would reach him.'

He shares the moving memory in the insightful new documentary “Super/Man: The Christopher Reeve Story” about his father’s life and legacy

Will added: “Surely a part of her died at that moment too.”

ABC correspondent Will was only three years old when his father was injured in the accident in 1995. He was 12 when his father died.

Sadly, his mother Dana died of lung cancer in 2006, despite never having smoked, just 17 months after Christopher's death.

Before her death, Dana had made sure Will was taken care of and arranged for him to live with his best friend's family while she underwent cancer treatment.

His older siblings also dropped everything to help him. Matthew worked as a producer while Alexandra Reeve studied law.

In the documentary, Will spoke openly about the heartbreaking loss he suffered as a young teenager, admitting: “Despite the love and security that my siblings, my family and my adoptive family gave me, that moment came, March 6, 2006… I have been alone ever since.”

In the years since their father's death, Will and his siblings have done everything they can to honor his legacy as one of Hollywood's most beloved stars.

Only a few years after graduating from the Juilliard School, where he studied Robin Williams – Christopher landed the role of Superman despite having only had a few roles and catapulted himself into worldwide fame.

While he became famous for his portrayal of a superhero on screen, his behavior after his own tragedy exuded heroism.

Will explained in a clip shown on Good Morning America on Tuesday: “I told him I love him. I'm going to do everything I can to make him proud.”

Superman star (left) Christopher was a real superhero before and after his paralysis in 1995. He died of heart failure in 2004 (pictured around 1977).

All three of his children work with the Christopher & Dana Reeve Foundation, whose goal is to cure spinal cord injuries by promoting innovative research and improving the quality of life for people and families affected by paralysis.

The documentary then celebrated its world premiere at the Sundance Film Festival in January, where it was acquired by Warner Bros. Discovery for $15 million.

Now the film will be released in a two-day special screening at Fathom Events, with the first showing on September 21 and the second on September 25, Reeves' 72nd birthday.

Will shared his pride in the project with GMA host Robin Roberts: “There's a universal story here. It's not about a famous person. It's about a family. It's a human story. We had human parents who did superhuman things.”

The new film gives a rare glimpse into his life behind the scenes, including his complicated relationship with his father, a professor, and the separation from his partner Gae Exton, with whom he had his first two children, Matthew and Alexandra.

It also shows how Dana struggled to keep her husband and family alive during the difficult times following his accident.

During a previous appearance on Good Morning America, Will read an excerpt from one of his mother's diaries.

The film legend's three children, Matthew (44), Alexandra (40) and Will (32), have worked together to present his life on the big screen

The entry revealed her deep pain over the loss of Christopher in 2004.

“I found this in one of her journals – 'I studied the difference between loneliness and being alone. I told my life story to the clean white towels I took warm from the dyer and pressed to my chest, a sad substitute for a body pressed close to me,'” Will read, explaining that his mother sought comfort in warm towels after they came out of the dryer.

He continued reading: “What I miss most, even now, are his hands, their expressive grace and their weight. The warmth of his hands on my skin, the embrace, two becoming one.”

“I carry the stack of towels upstairs, holding them carefully so they don't fall down. Except for one that is still damp, the top one that I had pressed against my face and that needs more time to dry.”

“She had to endure that,” said Will after finishing reading the entry.