The Haven't Met You Yet hitmaker was moved by the story of six-year-old Griffin Woods, who suffers from a severe form of autism and has become a big fan as his parents use the crooner's tracks to soothe him.
Buble recorded a personalised message and jingle for Woods and posted the clip on the internet.
In the video, Buble is sat a keyboard and says: "Hi, this is a YouTube exclusive. Truth is, this is only for one dude. It's for Griffin.
"Griffin, if you're watching, it's Michael Buble and I hear that you like watching my stuff on YouTube, so I wanted to do a song... It's a terrible song, I'm not very good on the piano but I love you man..."
The youngster's mother Christin, from Calgary in Canada, said: "I was laughing and crying at the same time. I was in disbelief... I couldn't believe it. My husband was crying. We showed our therapists and they were crying."