A statement released to Facebook on Friday afternoon by Michael, 41, and his actress wife Luisana Lopilato, 29, confirmed the diagnosis and requested privacy and prayers.
Michael wrote: "We are devastated about the recent cancer diagnosis of our oldest son Noah who is currently undergoing treatment in the US. We have always been very vocal about the importance of family and the love we have for our children.
"Luisana and I have put our careers on hold in order to devote all our time and attention to helping Noah get well. At this difficult time, we ask only for your prayers and respect for our privacy. We have a long journey in front of us and hope that with the support of family, friends and fans around the world, we will win this battle, God willing."
A similar statement was posted to Luisana's own Facebook page.
Michael and Luisana are also parents to son Elias, born in January.
Michael was recently in the UK to promote his new album, Nobody But Me, and record the BBC special Buble at the BBC, which just aired on Thursday night, the evening before he announced the heartbreaking news about his son.
In October, Michael told Entertainment Tonight that being a father to Noah and Elias was the most important role in his life, adding: "The only regret I've ever had is that I didn't do it earlier."
"I love them more than I'll ever love anything, or anyone in the whole universe," he said. "It's hard to be able to quantify that level of love. I didn't know that level of love existed. But it makes me better at everything."
His Instagram account is filled with beautiful images of his family, including photos taken on Noah's third birthday in September, when Michael said: "Can't believe my little man is 3 years old today !! Happy birthday Noah /Spider-Man. Your mommy And I love you so much! #loveofmylife #proud #papa."