"Shenouda III suffered renal failure ... due to the diabetes he endured for years," Sheif Doss told CNN.
The leader of the Coptic Christian community for 38 years, Shenouda was in poor health off and on for many years. He traveled to the United States to undergo medical tests last year.
Egypt's population is roughly 9% Coptic Christian, according to the U.S. State Department.
The Christian minority has been the target of a number of high-profile attacks in the past several years, including the bombing of a major church in Alexandria last January that left at least 21 people dead.
"The funeral is expected to take place in two days, as massive preparations must take place first. It is a historical event and 2 million people are expected to attend the prayers. I don't expect violence though," Doss said. more