Gwen Stefani Wishes Son Apollo Happy 9th Birthday in Tribute with Blake Shelton: 'Love U So Much'

Gwen Stefani's son Apollo Bowie Flynn turned 9!

In a sweet tribute post dedicated to her youngest son on his birthday Tuesday, the "Hollaback Girl" singer penned a special message sharing just how much he means to her to mark the milestone.

Along with the message, she posted various photos and videos of Apollo in different stages of his life as he grew up set to her band No Doubt's song "Running."

"My world is so much bigger with u in it, happy birthday, apollo ♥️♥️ i love u so much," wrote Stefani, 53, in the post's caption.

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In one throwback video, Stefani showed off her then-infant son, wrapped in white blankets and a diaper, while in another, she showed him in his toddler stage, cuddling a dog. She also shared a photo of baby Apollo sleeping over her shoulder.

By far, the biggest picture in the collage was of her with husband Blake Shelton and Apollo all smiling at the camera.

In the snap, Stefani looked cheerful in a green blouse and dark-blue jacket as she posed beside Apollo, who also grinned while weaning a denim jacket with his name embroidered on it.

Wearing a black shirt and matching black hat with the word "LEGACY," Shelton, 46, posed right behind the mother-son duo.

RELATED VIDEO: Gwen Stefani's Sons Apollo and Zuma Look All Grown Up as They Pose with Blake Shelton at NASCAR

The latter photo appears to have been taken last month when Stefani and Shelton took Apollo and her middle son Zuma Nesta Rock, 14, to NASCAR's Busch Light Clash at The Coliseum in Los Angeles.

The couple shared photos posing with the boys alongside NASCAR legend Jimmie Johnson at the event, where Johnson, 47, and his wife Chandra Janway hosted friends to watch in the Legacy Motor Club suite.

In December 2022, Shelton opened up to PEOPLE about becoming a stepdad to wife Stefani's three sons. (Stefani shares Apollo, Zuma and Kingston James McGregor, 16, with ex-husband Gavin Rossdale.)

"They've taught me something about myself that I never knew: I'm more than just a country singer or a goofy guy," he told PEOPLE exclusively of helping raise Kingston, Zuma and Apollo.

Shelton continued, "I'm someone they actually lean on, and that's not a responsibility that I ever had and not something that I ever considered even being into."

"It's a different kind of self-worth. Maybe it's the opposite of self-worth because you put yourself way down on the rung, and they move up ahead of you," added the singer.

Shelton announced last year that he would be leaving The Voice — a decision he said largely stemmed from a desire to spend more time with his stepsons as they grow up.

"You don't know what your regrets are going to be until you look back one day. I feel like my regrets would be, if I stayed on this path, that I am missing out on some more important things in life — and for now, that's our kids," he said.