Blake Shelton has become a strong male figure to Gwen Stefani's three sons, Kingston, 13, Zuma, 11 and Apollo, 6

Gwen Stefani is sending her youngest son a special birthday message!

On Friday, the No Doubt singer helped to ring in her son Apollo’s sixth birthday with a sweet social media tribute, featuring boyfriend Blake Shelton.

In the adorable selfie, Stefani, 50, poses next to Apollo while Shelton wraps his arms around the birthday boy and shows off his Oreo-topped celebratory cake.

“6 years ago GOD blessed us w this little angel ? boy ??,” the “Hollaback Girl” artist captioned the photo, adding the hashtags “thank you GOD,” “happy birthday” and “APOLLO.”

In his own post, ex-husband Gavin Rossdale, 54, wrote, “my love turned 6 – endless magic endless joy endless nutella———apollo we all love you so much —-? xx”

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Over Stefani and Shelton’s four years of dating, the country star, 43, has become a strong male figure to Stefani’s three sons, Kingston, 13, Zuma, 11 and Apollo, multiple sources previously told PEOPLE exclusively.

While staying respectful of Rossdale’s role as their father, Shelton has also had a large part in raising the boys over the last few years.

“He’s so fun and has shown Gwen’s kids another side of life filled with adventure,” said a friend of the Voice coach. “The boys have really grown immensely by spending time with Blake.”

Bringing joy and laughter into their lives, Shelton is also more than happy to shoulder more responsibilities.

Credit: Steve Granitz/WireImage

“He is a good dad, actually,” Stefani said in early 2019 on the Today show. “He’s been helping me out a lot. I literally get to the point where I’m like, ‘You gotta get home—I need help!’ It’s hard. I got three boys.”

Early on in their relationship, Stefani and Shelton’s loved ones quickly became “one big family,” and over the holidays the couple and their extended family traveled to Oklahoma together for their annual holiday get-together.

“They’re all very like-minded and comfortable with each other,” added the friend.