The 23-time Olympic gold medalist couldn't be a more adoring dad.
Phelps, 32, posted a heartwarming snapshot of his little boy grabbing at his facial hair while the proud papa holds him in his arms.
"Happy bday @boomerrphelps !! I love you so much little man... may this be your best 2nd bday your heart dreamt of!" Phelps captioned the precious photo.
"And @boomerrphelps bday weekend continues! #matchingpjs," the 23-time Olympic gold medalist wrote.
Boomer himself even shared a sweet message on his special day -- sort of. The birthday boy actually has his own Instagram account (with more than 783,000 followers) which is obviously managed by his adoring parents, but the posts are written from Boomer's point of view.
"So I guess today is my birthday?!!" read a caption to a photo of the 2-year-old in an awesome panda bath towel. "It’s been so much fun playing in the pool and hanging out with my fam today! Time to take a nap and see what other amazing things are planned for later."
Happy birthday, Boomer!
For more from Phelps on his family and fatherhood, check out the video below.
Michael Phelps on Expanding Family, Son Boomer's Swimming Skills and Whether He'd Do Reality TV (Exclusive)