The former couple - who were married from 2002 until 2016 and are also parents to Kingston, 15, and seven-year-old Apollo - each paid tribute to Zuma on social media along with heartfelt messages for the teen.
WATCH: Gwen Stefani's son Apollo is hilarious as he reveals his mum's dirty habit
Gwen, 51, shared two snapshots of her son - one showing Zuma as a smiling baby and a second recent headshot. She wrote: "Happy 13th b day to my Zuma baby. We love u so much!! Gx."
Gwen took to Instagram to celebrate Zuma's birthday
Gavin, meanwhile, posted a clip showing Zuma testing his boxing skills out on a dummy figure. "My sweetest boy turns 13," he wrote. "@zumarossdale21 you - young man are the most precious part of my life - thanks for all the light you bring - also, head of security position is open."
Among those to comment on his post was Zuma's older sibling Kingston, who sweetly wrote: "Happy birthday little bro, I love you."
Proud dad Gavin shared a touching message for his son
Former couple Gwen and Gavin were married for 14 years before their divorce was finalised in 2016. The singer is also a father to model Daisy Lowe from his previous relationship with Pearl Lowe.
In a 2017 interview with the Sun, Gavin made rare comments about his split from Gwen and admitted it was "shocking for everyone".
Gavin is a proud father of four
"For me, it was about being solid [for our sons] and silent but respectful of Gwen. That's what it'll be forever," he continued. "Those days were the worst days ever. It's all a bit of a blur."
Gavin, 55, further explained: "We had 20 years together and that in itself is pretty incredible. It's a lifetime. I still think she's incredible. It was a lovely 20 years and we have three amazing children.
Gwen and Gavin were married from 2002 until 2016
"There were a lot of positives, and with time they'll become more and more obvious."
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