The 78-year-old "Angela's Ashes" author, who has been fighting skin cancer, is in a hospice and "his faculties (are) shutting down," said Malachy McCourt. "He is not expected to live."
McCourt - a former city school teacher who won the Pulitzer Prize for his memoir - was being treated for melanoma, the deadliest form of skin cancer.
He was doing well enough that his doctors at Memorial Sloan Kettering Cancer Center recently allowed him to return to his home in Connecticut.
"He was out and about, being active, doing talks and so forth," said Malachy McCourt, also a writer.
About two weeks ago, he contracted meningitis and his condition deteriorated rapidly.
Malachy McCourt said about one in 25 melanoma patients also wind up getting meningitis - "he was one of the unfortunate ones."
"He's still conscious, but his hearing has gone and his eyesight is going," Malachy McCourt said. "He's speaking less."
Frank McCourt, an unlikely literary star, taught English in New York public schools for decades including 17 years at Stuyvesant High School.
He shot to international prominence in 1996 with the publication of his first book, "Angela's Ashes," which became an instant best seller and won a fistful of awards.
The eloquent memoir tells the story of his impoverished youth in Ireland.
A second book, "Tis" continued his life story, picking up when he returned to New York, where he was born, as a teenager.
Frank McCourt's third book, "Teacher Man" chronicled his years in the classroom.