Story highlights Zach Sobiech was diagnosed at 14 with osteosarcoma, a bone cancer

Last May, Zach was given a year to live after doctors ran out of treatment options

Zach's song "Clouds" went viral on YouTube; now celebs have posted a response

Zach Sobiech, 18, passed away Monday morning, surrounded by family and friends.

In the past few weeks, donations to Zach's cause -- the Children's Cancer Research Fund -- skyrocketed, as did downloads of his song, " Clouds ," which at the time of his death had more than 2.9 million page views on YouTube.

"We'd like to thank those people who listened with their hearts and helped Zach bring his message and his music to the world," the Sobiech family said in a statement.

When he was 14, Zach was diagnosed with osteosarcoma , a type of bone cancer that mostly strikes children. His prognosis wasn't great. Last May, with no more treatment options, he was given a year to live.

Zach began writing music after his cancer was diagnosed. His mom, Laura, was cleaning the family room downstairs last year when she found a folded-up piece of paper. She unfolded it to make sure it wasn't a homework assignment.

"I read through it and then I realized what I was reading," she said.

It was his first song, "Clouds." Zach wrote it for himself and his family. He wrote many more in the following years.

"I fell down, down, down into this dark and lonely hole," he sings at the beginning of the song, his voice beautiful, mellow -- kind of reminiscent of Jack Johnson's.

When Zach's song was posted to YouTube last year it quickly went viral. Recently dozens of celebrities posted a response -- joining forces to lip sync to Zach's voice singing "Clouds."

"And we'll go up, up, up, but I'll climb a little higher," sang Jason Mraz, Anna Faris and Rainn Wilson.

"Go up in the clouds because the view's a little nicer up here, my dear," sang Bryan Cranston and Rachel Bilson, Ashley Tisdale and Colbie Caillat.

"It won't be long now," sang the Lumineers.

"If only," sang Sarah Silverman, "I had a little bit more time."

It was a hello to Zach, and a farewell. It was a collective expression of love and gratitude.

The motley crew of celebrities on the video was assembled by Wilson, an actor, and director/actor Justin Baldoni, who directed a short documentary about Zach for his compelling online reality series, " My Last Days ," which runs on Wilson's SoulPancake YouTube channel.

"Going out to see Zach in Minnesota was one of the greatest experiences of my life," said Baldoni. "To make the choice to be happy despite whatever sad or tragic circumstances you may be living through ... he is this old soul who inspires everyone he meets. You leave and you think: 'I want to be a better person.'"

Watch Baldoni's film and you can see why.

There's Zach, a fresh-faced Minnesotan teenager, in one of the opening frames, saying: "I want everyone to know: You don't have to find out you're dying to start living."

Zach, top, poses for a photo with his family.

He tells us at the beginning of the film: "You know most people live kind of in the middle, in between 'all your dreams come true' and 'you're dying,' and it's a very comfortable place to live. I'm living on the two extreme ends, so you have really, really good days and you have really, really bad days."

He lived on those extreme ends for the better part of the past four years. And how gloriously.

In his final days, Zach slept a lot. Once in awhile he was able to muster up enough energy to bring out his guitar and play.

"It's those times when we remember how it used to be," said his mom.

She told me about a trip the family took last year, "a sort of pilgrimage to Europe," with 10 intense days of sightseeing.

"We packed a lot into those 10 days," she recalled, "and maybe these past 3½ years have been like that pilgrimage. I haven't had the chance to let it sink in. I don't let myself go to that place of grief. There will be plenty of time to think about what we have lost later."

And maybe, she said, that is the lesson here: that no matter where you are in life, look at what you have and be grateful.

It's the kind of thing you hear all the time, but putting it into action takes some work.

"That's what Zach's always been really good at: recognizing what is good and being grateful for it," Laura said. "He taught all of us how it's done."

Taylor Engel, one of the YouTube commenters on Zach's video, writes: "Well, you got a 6', 220 pound man sobbing his face off here. Prayers and thoughts with you, Zach...I hope someday I can be half the man you've become."

So many of us felt the same way as we watched his video for "Clouds" -- grateful for having had him walk among us, albeit for a short time, as he sang: "And maybe someday I'll see you again. We'll float up in the clouds and we'll never see the end ..."