(CNN) -- Zach Sobiech is dying with grace, love, joy and optimism -- the kind that somehow makes us all feel more alive.
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 just a year to live.
Zach turned 18 on Friday. On Saturday he went to prom with his girl, Amy. His high school class graduates next month.
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 says.
It was his first song, "Clouds." Zach wrote it for himself and his family. He has since written many more.
"I fell down, down, down into this dark and lonely hole," he sings at the beginning of the song.
His voice is beautiful, mellow -- kind of reminiscent of Jack Johnson's.
When Zach's song was posted to YouTube it quickly went viral, getting more than 2.6 million page views.
Now there's a new video you ought to see. On Monday, dozens of celebrities posted a love letter 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," sing Jason Mraz, Anna Faris and Rainn Wilson.
"Go up in the clouds because the view's a little nicer up here, my dear," sing Bryan Cranston and Rachel Bilson, Ashley Tisdale and Colbie Caillat.
"It won't be long now," sing the Lumineers.
"If only," sings Sarah Silverman, "I had a little bit more time."
It's a hello to Zach, and a farewell. It's 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," says 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."
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 has lived on those extreme ends for the better part of the past four years. And how gloriously.
Zach's sleeping most of the day now. His evenings are better. The other night 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," says his mom.
She tells 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 recalls, "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."
"Right now ... right now I feel really grateful," she continues. "That is the core of what I feel. I still get up in the morning and look forward to the day."
And maybe, she says, 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. Putting it into action takes some work sometimes. But there are always things in life to be grateful for, no matter where you are.
Being grateful is the doorway to all sorts of other good things, says Laura.
"That's what Zach's always been really good at: recognizing what is good and being grateful for it. It's the first thing he seeks out, his starting point. 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."
I swallow a sob. Me too, I think as I watch the video.
At that moment, Zach sings: "And maybe someday I'll see you again. We'll float up in the clouds and we'll never see the end ..."