Donald Trump's Great-Nephew William, 25, Is Nonverbal and Uses a Wheelchair. Here's What William's Dad Wants You to Know

"William is the most courageous and inspirational person I've met," Fred Trump III tells PEOPLE of his son, after recently alleging that Donald said severely disabled people "should just die"

Fred C. Trump III has been hoping this moment would arrive for more than 20 years — a moment when he can leverage his polarizing last name to advocate for the disabled community on behalf of his 25-year-old son, William, who has complications from a rare genetic mutation.

Fred, a nephew of former President Donald Trump, recently entered the spotlight with the release of his book, All in the Family: The Trumps and How We Got This Way, in which he describes growing up in the Trump dynasty and details his experience raising a child with "complex" intellectual and developmental disabilities.

In one shocking excerpt from the book that immediately made headlines, Fred, 61, claims that Donald, 78, once told him to "just let [William] die" due to the cost of care, even after Fred brought advocates to the Oval Office to discuss how the government should be more supportive of disabled people and their families.

Donald's spokesperson has dismissed the many allegations and quotes in Fred's book as "completely fabricated," calling them "total fake news of the highest order," though Fred maintains that they're "real stories."

Related: Donald Trump's Nephew Says He Told Him Disabled Americans, Including Fred III's Own Son, 'Should Just Die'

The publicity from All in the Family, which undoubtedly got more attention due to Donald's controversial presidential run, has already opened the door for more promising conversations about disability rights than he had while his uncle was president.

"Civil rights and voting rights started with a movement," Fred tells PEOPLE. "If I have a national platform, which apparently I do right now, I think I have to start a movement."

Related: Donald Trump’s Nephew Fred Endorses Kamala Harris for President After Making Bombshell Allegations Against Uncle

Fred has wanted to improve the way that disabilities are viewed by the American public and handled in the U.S. health care system since he and his wife, Lisa, were thrust into uncharted territory after William's birth. The couple's first two children were born healthy, and until William arrived, there were no indicators that he would have a different experience than his older siblings.

"You talk about life-changing... when you have a child who has hundreds of seizures a day like William did, where did they come from? Why were they here?" he adds. "It literally took Lisa 15 years of investigation through the internet and such to find out that he had a genetic mutation called KCNQ2."

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Two years ago, William — who is nonverbal and uses a wheelchair — moved into a group home about 20 minutes away from his parents in Connecticut for around-the-clock care.

Soon after, Fred started to reflect on his experiences and how they fit into the larger Trump family's narrative. He wrote All in the Family with the idea that, in addition to having a memoir component, it could be used as a resource for other parents who feel like they're drowning trying to adapt to a new normal.

"There's something I read years ago, pretty much 20 years ago. Someone said, 'Parents don't expect or want to have a child who's blind or who has a genetic mutation or who has autism,' " he says. "What my wife, Lisa, and I try to do in the book is let folks know what opportunities there are to gain knowledge on how to deal with the issues you're presented with that you didn't expect."

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Fred's book also expresses the need for disability rights, even at the highest levels of government. "[People with severe disabilities] are human beings," he tells PEOPLE. "They have thoughts and dreams and they have meaning."

Donald had a difficult time understanding William's health issues from the beginning, Fred claims in his book. In 2009, Fred — whose father died in 1981 — met with Donald to ask for help covering the costs of William's medical care and therapy.

"So what's the problem with him anyway?" Donald reportedly asked, according to the book. "Like what's wrong with him?" Fred writes that he explained how doctors believed William had a genetic disorder, but were still trying to pinpoint a diagnosis. Donald allegedly responded: “Not in our family. There’s nothing wrong with our genes.”

In the end Fred convinced all of his father's siblings — Maryanne, Elizabeth, Donald and Robert — to set up a joint medical fund for William.

Related: Donald Trump and Late Sister Maryanne Barry 'Cooled Their Relationship' in Recent Years: Sources (Exclusive)

During Donald's presidency, William says he visited the Oval Office about a dozen times for a variety of reasons. When his uncle's team connected him with officials at the Department of Health and Human Services to discuss his advocacy goals, he recognized that he was privileged to have a direct line to the nation's top decision-makers.

That insider access made it especially disheartening when, in May 2020, Fred says he learned that the commander-in-chief had been unmoved by his and other disability advocates' emotional presentation about making smarter investments to support disabled people and caretakers. After the meeting, Fred alleges in his book, Donald pulled him aside and said, "Those people... The shape they're in, all the expenses, maybe those kinds of people should just die."

Related: Donald Trump Refused to Be Seen with Military Amputees: 'Doesn't Look Good for Me,' His Former Chief of Staff Confirms

Donald allegedly repeated a similar, more pointed line later on when William's medical fund began running low and Fred called his uncle to ask for continued financial support. According to Fred's book, which the former president disputes, Donald responded, "I don’t know. He doesn’t recognize you. Maybe you should just let him die and move down to Florida."

Clearly taking issue with Donald's assessment, Fred tells PEOPLE, "William is the most courageous and inspirational person I've met ... He needs assistance with everything he does, but there's somebody within him. There is."

Fred says that only a few weeks ago, a spokesperson for Donald alerted him that William's medical fund had been dissolved.

With an elevated profile from his memoir, Fred is more determined than ever to affect change in the disability space, and he's putting pressure on elected officials and political candidates to help. "I'm not just going to be a face," he says of his new platform. "I'm going to be a mouth."

Fred has three main areas that he hopes to see improved: caregiver training and funding; housing options; and prevention, by training medical professionals to identify and treat conditions early on in a child's life. He has already met with some lawmakers, and recently he spoke at an event for the American Association of People with Disabilities.

After trying — and failing — to make inroads with his uncle's administration, Fred announced that he would be backing Vice President Kamala Harris in the 2024 election. But while he has expressed a willingness to campaign for her and was even invited as one of her guests at the Democratic National Convention in August, she won't get a free pass.

"I've made it very clear to [the Harris campaign] at high levels that disability advocacy is super important to me and I'm not letting it go," he said. "Whether it's a potential Trump administration or a potential Harris administration, we're going to hold their feet to the fire."

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