September 02, 2016 10:55 AM

Andrew Lipovsky/NBC/NBCU Photo Bank via Getty

Michael Phelps‘ son Boomer Robert has been getting almost as much attention as his dad lately.

Between sweet photos taken in the stands at Phelps’ last Olympic competitions to having over half a million followers on Instagram, the newly retired swimmer’s son — who turns 4 months old on Monday — is quite the popular one.

But how did Phelps and his fiancée, former Miss California USA Nicole Johnson, come up with their first child’s name?

“We heard the name, and we didn’t want, like, a normal name,” he told Jimmy Fallon on Thursday night’s episode of The Tonight Show. “And I brought it up to Nicole and she was like, ‘Oh, that sounds kinda cool.’

“Boomer, like boom boom … we didn’t know either. We had no second option — once we picked that, we were like, ‘Yeah,’ ” Phelps, 31, adds with a laugh. “He’s definitely gonna be the cool kid.”

Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Babies newsletter.

Boomer is already pretty cool, if his social media following is any indication. Phelps says he runs his son’s account, and Johnson takes a lot of the photos and sends them to Phelps when they’re not together.

“He went from 5,000 followers in late June to … I think I just looked, and he has like 660,000 followers,” the proud dad says. “It’s insane. He’s gotta be the most followed baby in the world!”

“I jumped into the photo [backstage] because I had to grab him, I had to smell his little head,” Fallon says.

“There’s nothing better than a baby smell, it’s incredible,” Phelps adds.

Phelps shares that he and his family live in Arizona, and that they have a pretty busy rest of the year coming up between volunteering for the Arizona State University swim team, traveling “here and there,” charitable work — and planning a wedding.

“We’re getting married at the end of the year,” he shares to applause and cheers from the audience. “We have a very busy year. But we’re excited and pumped, and the little man gets to travel everywhere with us, so it’s pretty awesome.”

Jen Juneau

You May Like