JJ became a father for the first time when his model girlfriend Caterina Lopez gave birth on 28 November last year.
X Factor star JJ - who makes up one fourth of the pop group alongside Josh Cuthbert, Jaymi Hensley and George Shelley - opened up to Reveal about fatherhood at Capital's Summertime Ball with Vodafone this weekend (21 June).
"Fatherhood is amazing, it's really good," says JJ. "Princeton's nearly seven months old. It's gone really quick but he's a little diamond. He's just a little cutie."
And have JJ's bandmates been embracing their roles as uncles? "They've been amazing uncles," he tells us.
Union J rose to fame after starring on The X Factor in 2012, in which they were mentored by Louis Walsh. The boys finished in fourth place.
Auditions for this year's series has just kicked off, and with the likes of Cheryl Cole, Simon Cowell and Mel B joining Louis on the panel, the boys reckon X Factor will have another successful year.
"I think even if it was just Simon coming back, he would make a massive difference," explains Josh. "Now it's got the recipie to be the best show and to get The X Factor back to where it belongs."
Following their success on the ITV talent show, Union J have achieved two top 10 singles, a top 10 album and have embarked on a UK headline tour.
When we grill the boys about their strangest fan requests, Jaymi reveals: "While we were doing X Factor, Josh got asked by a woman if he would make her and her daughter pregnant. That was weird."
Recalling the moment, Josh adds: "The poor daughter was going, 'Stop it mum'. I was just like, 'No'."
Fan requests aside, the boys are now busy working on their second studio album and reveal that they would love to collaborate with Iggy Azalea in the near future.
"We'd love to collaborate with Iggy," says JJ. "She's doing so well in America and to claim the top two spots in the music charts over there is just insane."
Speaking of the album, Jaymi says: "We're going to get the album out by the end of the year and we've recorded so many songs for it; around 30 so far. There's a few we've done with collaborations in mind but nothing's set in stone yet - fingers crossed though."
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Watch Union J's video for 'Loving You Is Easy' below: