"Everything shuts down around you," Foster says. "You go inside your little bubble."
The music producer, who has been a mentor to Buble over the years, doesn't think the 41-year-old singer is checking social media during this trying time but is "sure he feels the love of millions of people."
"He's in good shape, he's got good doctors around him and we will hope and pray for a great outcome, which I believe they will have," Foster adds. "You get in a bubble and nothing else matters."
Earlier this month, Buble and his wife, Luisana Lopilato, shared the sad news about their son on Facebook and told fans that they would be putting their "careers on hold" until Noah was better. "I don't think he'll sing again until his child is well," Foster said. "I am certain of it."
"He's done a lot of good for a lot of people," he says of his friend. "And I think this is going to have a great, fantastic ending to it and be a new beginning."
ET recently spoke exclusively with Buble -- who just released his seventh studio album, Nobody But Me -- about the love he has for his sons, and how fatherhood changed him. "I didn't know that level of love existed, but it makes me better at everything," he said of Noah and his 9-month-old son, Elias. "It allows me to pull more emotion and be more honest and to be in the moment."
Buble also opened up about the fears that come with being a parent. "You're scared all the time because you love them so much that you can't ever, ever think of a world without them," he admitted. "You try not to even think about it."