Prince Frederik, the youngest son of Prince Robert of Luxembourg and Princess Julie of Nassau, has died. He was 22.

Frederik died on March 1 from the rare genetic condition POLG Mitochondrial disease, his father announced in a heartfelt message posted on the website for the POLG Foundation, an organization founded by Frederik to help find treatments and a cure for the illness.

“It is with a very heavy heart that my wife and I would like to inform you of the passing of our son,” Robert, 56, began his lengthy statement.

Detailing that Frederik “called us in to his room to speak to him for one last time” on what marked “Rare Disease Day” on Feb. 28, the royal explained that his son “found the strength and the courage to say goodbye to each of us in turn — his brother, Alexander; his sister, Charlotte; me; his three cousins, Charly, Louis, and Donall; his brother-in-law, Mansour; and finally, his Aunt Charlotte and Uncle Mark.”

“He had already spoken all that was in his heart to his extraordinary mother, who had not left his side in 15 years,” Robert continued of wife Julie, 58.

Prince Robert of Luxembourg and Princess Julie of Nassau with their three children, Prince Frederik, Prince Alexander and Princess Charlotte. Grand Ducal Family of Luxembourg/Instagram

According to Robert, Prince Frederik gave varied goodbyes, “some kind, some wise, some instructive,” and, “in true Frederik fashion, he left us collectively with a final long-standing family joke.”

“Even in his last moments, his humor, and his boundless compassion, compelled him to leave us with one last laugh, to cheer us all up,” the proud dad continued.

Prince Robert then detailed that his “beloved son” asked him a final question “prior to his other remarks” before his death: “Papa, are you proud of me?”

“He had barely been able to speak for several days, so the clarity of these words was as surprising as the weight of the moment was profound. The answer was very easy, and he had heard it oh so many times, but at this time, he needed reassurance that he had contributed all that he possibly could in his short and beautiful existence and that he could now finally move on,” Robert said.

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“Frederik knows that he is my Superhero, as he is to all of our family, and to so very many good friends,” Robert also said in his statement, later adding that his son “was born with a special capacity for positivity, joy, and determination.”

“When he was little, I would always say that if there is one child of ours that I would never need to worry about, it was him,” he continued. “He has social skills like no other, an amazing sense of humor, an emotional intelligence and compassion that were off the charts, a sense of justice, fairness and decency that knows no bounds. He was disciplined and organized beyond belief.”

Robert then stated that Frederik was “particularly headstrong (at times, I might have used the word stubborn).”

“As his siblings and most anyone who ever met him say, ‘Frederik is the strongest person that we know!’ ” the royal further explained. “Frederik fought his disease valiantly until the very end. His indomitable lust for life propelled him through the hardest of physical and mental challenges.”

Prince Frederik with his siblings, Prince Alexander and Princess Charlotte. Grand Ducal Family of Luxembourg/Instagram

Frederik was born with POLG Mitochondrial disease, his father wrote, though his family did not know it at the time.

He was later diagnosed at age 14, “when his symptoms were showing more clearly and when the progression of his disease had become more acute.”

The disease, according to the POLG Foundation, “is a genetic mitochondrial disorder that robs the body’s cells of energy, in turn causing progressive multiple organ (brain, nerves, liver, intestines, muscles, swallowing and ocular function, etc.) dysfunction and failure.”

Despite Robert describing Frederick’s illness as a “burden that he had to carry throughout his life,” the royal, who is the first cousin of Grand Duke Henri of Luxembourg, said his son always handled uphill battles “with grace and with humor” and was beyond proud of the work he did with the POLG Foundation.

Robert pled with those reading the tribute to donate or volunteer with the organization in memory of his young son.