The No Doubt star took to the social media site to share a snap of her teenage son playing a guitar, writing, "happy 15th b day to my beautiful boy @kingsrossdale_ love u mom gx."
Meanwhile, her ex-husband Gavin expanded on his love for his eldest child in his post, which was accompanied by a shot of Kingston as the sun set.
"HAPPY BIRTHDAY KING - 15 years of life's best journey – you are my best focus and inspiration - you are smart funny and kind with oceans of empathy - to me that's all you need to make your mark - i could not be more proud of you - and i love you more every day - i am in awe of you - shine on - and yes i feel a lovely responsibility (huge) with your development - shout out to all my the single parents who are love joy and survival - my sweet boy - @kingsrossdale_," he wrote.
Gavin and Gwen share three sons - Kingston, 12-year-old Zuma and 7-year-old Apollo. The former couple finalized their divorce in 2016, eight months after she submitted legal documents to end their 13 years of marriage.
While he and his ex continued to co-parent their children, Gavin admitted in April 2020 that COVID-19 lockdown complicated their split custody agreement. To SiriusXM's "Trunk Nation", he admitted, "I know who's around me and know who's bringing the corona - no one - but you send your kids out and now they're coming back to you, and now you're prone to whoever they're with. So, it's a tricky one with all divorced parents."