The Spartacus star, 94, tells People.com he broke down in tears of joy after learning the actor's tumour has gone, six months after he was diagnosed with stage four throat cancer.
He says, "I immediately felt years younger (after hearing the good news). Not a day over 85! I believe God answers all prayers. Sometimes his answer is no, but not this time."
And Douglas has heaped praise on his Wall Street star son for managing to remain "gracious" while undergoing gruelling radiation and chemotherapy treatments.
He adds: "I admire the way Michael has handled this personal crisis. He was open and honest with his family, the media and the public. He was gracious throughout, trying to live a normal life despite the constant press scrutiny and daily treatment."