Joaquin Oliver was one of the 17 students and faculty murdered on Valentine's Day at Marjory Stoneman Douglas High School in Parkland, Florida, when a former student allegedly opened fire with an AR-15-style rifle.
On Sunday, his fellow seniors will walk across the stage at the BB&T Center in Sunrise, Florida, to accept their diplomas.
But Joaquin and three other seniors gunned down -- Nicholas Dworet, Meadow Pollack and Carmen Schentrup -- will not be there.
Joaquin’s parents, Manuel and Patricia Oliver, sat down with ABC's "Nightline" remembering their 17-year-old son's "deep" mind and open spirit.
Nearly four months later, the mourning parents continue to grapple with the loss.
"You have no idea ... what it feels to get back home and not have your son here," Manuel said, choking up. "And not having anybody to say 'Hi!' to.
"It's an empty space. And it's sad. And you cry a lot. And you don't sleep well. And you try to fight for an answer that is not there yet."
The Olivers have since founded a nonprofit organization called Change the Ref, a platform inspired by their son in part to educate and empower youth in the movement to end gun violence.
“This is not about us. At some point, you find yourself in a situation that is not anymore about yourself,” Manuel told "Nightline."
Manuel said the nonprofit’s name came from a conversation he had with Joaquin, a few months before he died.
Joaquin was flustered by a series of bad calls a referee had made in a basketball game. The frustration inspired Joaquin and his father to call the recreational league and ask to have the ref switched for someone who was fair.
“He started this fighting way before we started,” Manuel said of his son. “After the tragedy, I understood that conversation. And that conversation was ... demanding a fair game."
Since the shooting, that idea has transcended beyond the basketball court and into the political realm.
“And when I saw that some politicians were unable to discuss about gun control, which is one of our main issues, because they are attached and receiving money from the [National Rifle Association], that is exactly when I found out what that name meant,” he continued. “So, Change the Ref is looking for the right ref to make the right calls, that he has to be impartial, he cannot have any connection or any kind of personal interest with any one of the teams.”
Manuel told "Nightline" that he literally -- and figuratively -- had to take a walk in his son’s shoes. He started wearing his son's basketball sneakers in the days and months after his death.
"It's a way to follow a path ... that he started," he said.
In a light moment with "Nightline" producers, Manuel joked, "Also, we have the same taste ... because he could have left ugly shoes and that wouldn't be so cool."
Joaquin's parents have started using art as a silent weapon for change.
Manuel wants his son's likeness and others slain in school shootings to have a voice. He paints murals demanding change.
“I’ve been painting my whole life,” he said. “Joaquin is right there with me painting. I became a tool that Joaquin can use to still have a voice. A very loud voice, a very specific voice, a very Joaquin voice. The way that these walls are painted and treated in that graffiti, street art style, that is totally my son.
“It’s not me being an artist; it’s Joaquin being an activist. That’s exactly what the walls are,” he added.
The Oliver’s have taken these murals to places like Springfield, Massachusetts, the home of Smith and Wesson, and Dallas, the site of this year’s NRA convention. This summer they will build a mural in Chicago, where high rates of gun violence has frustrated officials for years.
The murals have been signed by thousands, some even taking a piece of it home.
"They feel they're a part of it, and that's an amazing feeling," Manuel said.
Manuel is heartened that their creative movement contributed to growing conversations on policy, which he hopes will affect the upcoming midterms and 2020 election.
“We have to go and vote and raise our voice,” Patricia said of the movement.
Joaquin's parents said one of their main goals is to erase the long-standing financial relationship between U.S. politicians and the NRA.
“That is going to be a long, hard fight but we are committed to that,” Patricia told "Nightline."
“I feel that he’s there for us, with us,” she said of her son. “He shows up in a different way.”
Manuel recalled how, the night before the shooting, he went with his son to buy flowers for his girlfriend, Victoria Gonzalez, for Valentine’s Day. Joaquin took extra time getting dressed for school the following morning and proudly held her flowers and card in the car as his father drove him to school.
“I said, ‘Love you.’ And he gave me a kiss, ‘I love you too.’ And I told him, just make sure you call me to see how did it go with the flowers,” Manuel said. “And then he never called me.”
Manuel said the fight to end gun violence “starts at home.”
“In another house that same night, something totally different was happening. Someone had other plans. Same Valentine’s Day, same day to celebrate friendship and love -- someone wasn’t buying flowers,” he said. “What happens in your house is under your control; it’s not under any politicians’ control.”
Just a few months ago, the parents were dreaming of the day when they would watch their children, Joaquin and Andrea, get married and raise families of their own.
“We had a lot of plans for our lives,” Manuel said. “These kind of events destroy any plan that you had and leave you in an empty space with no plan at all. And today, the only plan that we have is to wake up tomorrow and keep on fighting and survive for 24 hours so we can wake up again after tomorrow and keep on fighting.”
“This is a completely new book that we’re starting, the three of us,” Patricia told "Nightline."
“We [would] love to enjoy our daughter’s kids at some point, but we must have a safe place for them. We’re not taking this risk twice,” Manuel added. “You don’t have to pay the price that we paid to understand what’s going on. Nobody told me that, nobody gave me that advice. I wish somebody gave it to me and maybe, maybe I would have started my fight before. Just maybe.”
ABC News' M.L. Nestel and Samantha Sergi contributed to this report.