"I believe God answers all prayers," the 94-year-old screen icon tells PEOPLE. "Sometimes his answer is no, but not this time!"
Douglas says he cried tears of joy when Michael told him the news that the radiation and chemo got rid of the tumor in his throat. Describing his relief, the patriarch adds wryly, "I immediately felt years younger. Not a day over 85."
The Hollywood legend – who lost his 46-year-old son Eric Douglas in 2004 to an accidental drug overdose – also made a point of praising his son and daughter-in-law Catherine Zeta-Jones.
"I admire the way Michael has handled this personal crisis. He was open and honest with his family, the media and the public," says Douglas. "He was gracious throughout, trying to live a normal life despite the constant press scrutiny and daily treatment."
As for Zeta-Jones, she "was wonderful," says Douglas. "I adore her and the beautiful life she has given my son. I believe they will have many more years together."
Michael Douglas, 66, told PEOPLE, "The most difficult part was letting [my father] know, telling him [I had cancer]. There's something about when your children are seriously ill, it affects you more. So I was worried about him," he said.
The Wall Street Oscar winner told PEOPLE that the California-based Kirk traveled to New York three times, just to check on him. "He was quite extraordinary," he noted.
Over the past few months, Michael also admits, he heard from several friends and family members who may have seen tabloid photographs of him looking extremely gaunt.
"I did keep getting these calls, 'You sure you're okay?' " said Douglas, who lost 32 pounds during his treatment but has gained back about 12.
"Michael is a mensch," (Yiddish for "human being") says his proud father. "He worries about all of us more than himself. I don't think he got that from me. Maybe he got my longevity genes. That would make me very happy."