"Even in his last moments, his humor and boundless compassion compelled him to leave us with one last laugh...to cheer us all up," Prince Robert said.
He also revealed that Frederik, realizing his time was short, called his family members to his room "to speak to him one last time" on February 28, the day before his death.
His last messages were "kind, wise, and instructive," Robert continued.
Frederik was born with POLG Mitochondrial Disease, which wasn't initially diagnosed.