Zac Hanson Welcomes Son Quincy Joseph Thoreau with Wife Kate, Jokes That He's 'Freaking Out'

Welcome to the Hanson family!

Zac Hanson, the youngest brother in the musical group and the band's drummer, and his wife Kate welcomed their fifth child,son Quincy Joseph Thoreau, on Sunday, he revealed on Instagram.

"I am a little late, but I celebrated #internationalwomensday the best way I know how," Zac, 35, wrote on Tuesday, sharing a photo of his wife holding the newborn as well as a shot of himself jokingly "freaking out" over the baby boy's arrival.

"Marveling at the superpower women are uniquely gifted... as well as freaking out over the fact that I am now responsible to help raise another person," the dad continued. "Quincy Joseph Thoreau Hanson. Born 3/7/21."

Want to get the biggest stories from PEOPLE every weekday? Subscribe to our new podcast, PEOPLE Every Day, to get the essential celebrity, entertainment and human interest news stories Monday through Friday.

Zac married his longtime love Kate Tucker in June 2006 after five years of dating. They also share four other children together: Mary Lucille Diana, 4, George Abraham Walker, 7, Junia Rosa Ruth, 10, and John Ira Shepherd, 12.

The parents announced their pregnancy news exclusively with PEOPLE in January.

"We're so happy and thankful for the things 2020 got right, especially this little guy coming in March! 💙 Here's to fresh starts and new beginnings," Zac said at the time.

Celebrating Kate's birthday in November, Zac wrote on Instagram alongside a selfie: "Happy birthday 🎉 🎂 to my favorite person in the world @kathrynthanson just as beautiful as the day we met 20 years ago."

Zac and Kate's latest addition is the 15th child among the three brothers in the Hanson band, which also includes Taylor, 37, and Isaac, 40.

RELATED GALLERY: Celebrity Babies Born in 2021 (So Far!)

Taylor and wife Natalie, 36, welcomed daughter Maybellene Alma Joy on Dec. 7. The couple also share daughters Wilhelmina "Willa" Jane, 8, Penelope "Penny" Anne, 15, and sons Claude Indiana Emmanuel, 2, Viggo Moriah, 12, River Samuel, 14, and Jordan Ezra, 18.

Isaac and his wife Nicole have three kids: Nina Odette, 6, James Monroe, 12, and Clarke Everett, 14.

"It's a cool thing," Zac said of their collective kids becoming close over the years, while appearing on the Lorraine talk show in February 2019. "We spend a lot of time together touring, in the studio and around each other, so our kids know each other really, really well. They're really, really close."