Robin Williams was the first friend in the hospital after Christopher Reeves' accident (exclusive)

• In PEOPLE's latest cover story, Christopher Reeve's children recount their father's extraordinary life, including his friendship with Robin Williams
• The new documentary Super/Man: The Christopher Reeve Story examines how much Robin Williams' humor saved Christopher Reeve after he was paralyzed from the neck down in 1995
• Will Reeve says Robin Williams was the first friend to show up at the hospital after his accident and made him laugh by pretending to be a Russian proctologist

Will Reeve talks about his father Christopher Reeve's unique friendship with the late Robin Williams.

“Robin was dad's best friend, and you're there for your friends,” Will, 32, tells PEOPLE of the longtime friends, who met in the early '70s when they were theater students together at Juilliard. He says they became so close that they called each other “brother.”

“Our father and Robin had a unique bond,” says Will. “They had a friendship that someone should make a movie about, but what came through was just their love and respect for each other, and that never wavered.”

Will notes that Williams was the first friend to show up at the hospital in Virginia after he fell off a horse and was paralyzed from the neck down. Williams not only showed up, but made Christopher laugh by pretending to be a Russian proctologist.

“No one knew how to show love and the right amount of humor better than Robin Williams and his wife Marsha, who we call our fairy godmother,” says Will, adding, “We are still incredibly close to her.”

In the new documentation Super/Man: The Christopher Reeve Story (produced by Words +
In “The 40 Fingers” (2012), which opens in select theaters on September 21 and 25, their friendship is portrayed using old film footage.

The film also paints a comprehensive portrait of Christopher's life before and after his accident, including his early days as an actor when he first became close to Williams.

In his 1998 memoirs Still meChristopher remembered a first impression of the Mrs. Doubtfire actor, who wrote that when they met, Williams “wore tie-dye shirts with sweatpants and talked like crazy. … He was like a balloon inflated and then released. I watched in awe as he practically bounced off the walls of classrooms and hallways. To say he was 'on' would be a huge understatement.”

Will says that after his father's accident, it wasn't just Williams who stood up for the family.

“We were fortunate to live in a close-knit community and had a large circle of friends, teachers and coaches, and people from the past and present always helped us wherever they could,” Will recalls.

But the presence of Williams and Marsha was particularly impressive. “Robin and Marsha have done so much for us, things we can never repay. We are eternally grateful to them for the way they have supported and loved us and continue to do so.”

Tickets for Super/Man: The Christopher Reeve Storyvisit Fathom Events.

To read more about the extraordinary life of Christopher Reeve, pick up the latest issue of PEOPLE on newsstands Friday.