Michael and his wife Luisana had taken their son to the US where they raised concerns over his health thinking he had mumps.
It was then discovered their little boy had liver cancer.
Doctors have allegedly told Buble that three-year-old Noah has a "90 per cent" of surviving after having four months of chemotherapy.
Luisana's sister Dianela told Gente magazine: "If God wanted it to be discovered that way, it's because he wants to save Noah.
"Cancer is a horrible illness but we have faith in Noah being able to overcome this.
"We know he will. The time to start a long and very difficult treatment has come but it's got to be done.
"They say that children are often strong enough to withstand chemotherapy... and my nephew is a warrior."
It comes as Michael and Luisana released a statement confirming they have put their careers on hold.
The statement read: "We are devastated about the recent cancer diagnosis of our oldest son Noah who is currently undergoing treatment in the US," the statement read.
"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.”
Close friend and pianist David Foster has revealed that Michael will not return to music until Noah is well again.
He said: "He's in good shape. He's got good doctors around him.
"We will hope and pray for a good outcome, which I believe that they will have."
Despite the support from family and friends, the 67-year-old said that Michael "will not sing again until his child is well".
He added: "He's a good man. He's got a lot of good karma coming his way. He's done a lot of good to a lot of people."