Paul Cantlie read the statement from his hospital bed after undergoing throat surgery.
"To those holding John, please know that he is a good man," he said.
He said his son was an "impartial and respected journalist" who wanted to tell the world about Syria, "for the good of the people, as a friend and as a civilian."
"I want John to know how very proud I am of him," the 80-year-old said. He urged his son's captors "to help us and allow him to return safely to those he loves and those who love him."
Also Read
Cantlie was previously held by militants for several days in July 2012 along with a Dutch journalist.
Cantlie's captivity, and that of other Western hostages, was kept secret for months out of concerns publicity would endanger their security.
In recent weeks, the militants have released online videos showing the beheading of two American journalists and a British aid worker, and threatened to kill another Briton, Alan Henning.
He said family had been trying to communicate with the militants to deliver "an important message regarding John" but had received no response.