Stefani, 50, posted a video on Instagram of Shelton, 43, showering Kingston with kisses while celebrating the occasion Tuesday.
The clip shows Shelton embracing Kingston as he smooches him repeatedly on the cheek. Kingston smiles before escaping and wiping his face.
"happy b day kingking gx @blakeshelton," Stefani captioned the post.
Stefani also shared a birthday tribute to her eldest son.
"happy 14th bday to my first born son- thank u God for marking me HIS MAMMA GX #loveukingstonjames," she wrote.
Stefani has three sons, Kingston, Zuma, 11, and Apollo, 6, with her ex-husband, singer and musician Gavin Rossdale. Stefani and Shelton started dating in 2015 following their respective splits from Rossdale and Miranda Lambert.
"We're not engaged. We're not married. And he's my boyfriend... still," she said.
Stefani, Shelton and other celebrities took part in NBC's Red Nose Day special for charity last week. The event raised funds to help end child poverty.