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Michael Phelps shared the sweetest pic to celebrate son Boomer's birthday

See the heartwarming throwback photo Michael Phelps shared to celebrate his son Boomer's first birthday.
/ Source: TODAY

Happy birthday, Boomer!

Michael Phelps celebrated son Boomer's first birthday on Friday with a sweet throwback photo to the day of his birth, writing on Instagram that "for me it's the best thing that ever happen (sic) to me!!"

Since stealing the spotlight from his record-setting dad during his final Olympics in Rio de Janeiro last summer, Boomer has given the legendary swimmer a crash course in fatherhood.

"Nobody can really prepare you for it, you're going through it and you're learning as you go," Phelps told TODAY Parents last month. "Watching (wife Nicole) as a parent, watching her as a mom, she's the best. She's always reading things and just trying to make sure that his life is the best that we can make it."

Boomer became a mini-celebrity while being held in the stands in Rio by Nicole as his dad capped his career with an Olympic-record 23 gold medals.

Since then, he's hit milestones, like drinking from his own cup and clapping along with an audience, which Nicole detailed in a sweet Instagram post of her own on Thursday.

The family is planning an around-the-world theme for his birthday party, considering he's already traveled far and wide in his first year.

"It's wild to think that a year has passed," Phelps said. "Nicole and I will still look at each other like, 'We have a child! This is our child.' It's probably the best thing I've ever, ever been able to be a part of."

And for all future Olympians, look out. Boomer is also learning how to swim.

RELATED: Michael Phelps on learning to be a dad to son Boomer

Follow writer Scott Stump on Twitter.