When Nicole Powell went to bed after posting to Facebook a cute photo of her 6-year-old son singing to his baby brother, it had 10 shares.

"Oh, that's nice," she said, recalling her thoughts to "Good Morning America."

But the next morning when she woke up, the post exploded -- and had been viewed more than 1 million times.

Tripp, the 6-week-old brother in the Cabot, Arkansas, family of five children, was born with Down syndrome and spent the first weeks of his life in intensive care.

He came home on Christmas Eve, his mom said.

Every day while he was in the hospital, his family, including big brother, Rayce, would visit.

"From the minute [Tripp] was born, Rayce was like: 'Hand me the baby,'" Powell recalled. "Each day after school he would just talk and talk to Tripp, telling the baby all about his day."

Eventually, the proud big brother picked a "song" for his baby bro, Powell said -- "10,000 hours" by Dan + Shay and Justin Bieber.

"Every time it would come on he would just tell Tripp, 'This song's for you,'" his mom told "GMA."

Powell finally captured Rayce singing to baby Tripp on video.

"I'd spend 10,000 hours and 10,000 more, oh, if that's what it takes to learn that sweet heart of yours. And I might never get there, but I'm gonna try. If it's 10,000 hours or the rest of my life I'm gonna love you," the 6-year-old sings.

Powell said most of the comments say what a great big brother Rayce is.

"It' so so sweet," she said, "but I already knew that."