Liam Payne Jokes Son Bear Is 'Humongous': 'I'm Going to Get a Sore Back!'

Liam Payne’s son is growing in one direction — up!

“He’s just big in general — he’s humongous,” Payne told PEOPLE on Tuesday about his 4-month-old son Bear Grey at the Musical.ly headquarters during a ‘muser mingle,’ where he launched the hashtag campaign for his new song “Strip That Down” for the entertainment social media platform. “I’m going to get a sore back. He’s a big boy! He’s healthy, he’s strong, he likes his bouncer.”

The boy band member-turned-solo singer recently took to Instagram to share a sweet photo of him and Bear wearing a matching pair of black-and-white Yeezys. But great style isn’t the only thing the father-son duo have in common.

“I thought he looked like me, but then I saw a picture my mom sent me of myself when I was a baby and I was like, ‘Whoa,’ ” Payne shares. “I was a chubster baby.”

Although Bear has yet to hit any major milestones, the drool is beginning to pour at a rapid rate. “I’m just going home towards the start of what could be the terrible teething,” the first-time father says.

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The former One Direction singer and his girlfriend, Cheryl, welcomed their first child together in March — yes, girlfriend.

“In the U.K., you say ‘the Mrs.,’ ” Payne says, clearing up the rumors that he and Cheryl have not tied the knot despite much speculation from fans after he referred to her as his wife during a handful of interviews. “Maybe one day.”

For now, Payne is enjoying life as a new father and focusing on his solo career. Aside from his debut solo track “Strip That Down,” which is quickly climbing the charts, Payne collaborated with deejay Zedd to produce a summery club-pop hit called “Get Low.”

“We’re trying to figure out what to do with the video for ‘Get Low,’ ” he says, having already released the dance video for “Strip That Down.”

“There’s got to be some sort of dance to it,” he adds. “We’ll be doing the ‘get low!’ ”

While Payne has been successful in putting out hits of his own in recent months, he will never forget where he started.

“We’re like the Avengers — you call us over miles and we’ll just arrive on time,” he says about maintaining a relationship with his fellow One Direction members.

“I saw Niall [Horan] a few weeks ago when I played a show with him in a horse corral, and then I saw Harry [Styles] not in a horse corral.”

He continues, “I am so excited to see [Harry’s] movie by the way! I’ve heard great reviews. He seems to be having a nice time. He works hard.”

Musical.ly hosts over 200 million users and has partnered with other artists in the past such as Selena Gomez, Gwen Stefani, Ariana Grande and Jason Derulo on similar campaigns to promote music discovery in the app and enrich the relationship between artists and their fans, who like to “spend a lot of time being creative and having fun,” Payne tells PEOPLE.