had a heartfelt message for someone very special on Tuesday when her , turned nine. 

The singer shared photos and a sweet tribute to the little boy on Instagram when she wrote: "My world is so much bigger with u in it, happy birthday, apollo. I love you so much."

Gwen added a pair of red heart emojis and fans rushed to wish her firstborn a happy birthday too. 

WATCH: Gwen Stefani and Blake Shelton's incredible love story - from heartache to happiness

There were several images shared, including one of Gwen's husband with his stepson. 

While they don't have any children together, the country music star has been open about how much he adores being a stepdad to Apollo, Zuma, 14 and Kingston, 16. 

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, is still a huge part of their lives, but Blake is also determined to be the best parent he can be. 

Gwen shared the montage of her son for his birthday 

"There's definitely nothing easy about it," he said during an interview on radio show . "I don't know if it's as hard or harder or not as hard as being an actual biological parent, you know?"

He continued: "I'm not gonna lie. I don't take it so serious that I'm not enjoying this time because I really am," he continued. "Especially, you know, now that we're five years into this thing, I can't imagine my life without these kids now."

Blake and Gwen are a very happy family with her three sons 

So much so, after 23 seasons to make more time to be a present stepfather. 

"If I walked away from my career at this time, the only thing that I run the risk of is having regrets that I'm missing out on some more important things in life," he told . "For now, that's our kids. This isn't about me anymore and never will be again."