The 48-year-old singer and 52-year-old Bush frontman dedicated sweet posts to their "little angel" on his fourth birthday Wednesday.
Stefani shared a photo on Instagram of Apollo from his Scooby-Doo-themed birthday party. The picture shows Apollo wearing face paint and his Scooby-Doo onesie.
"#happybirthday to this little angel boy - #senttomefromheaven #truemiracle #4yearsold #greatful," she gushed in the caption.
Stefani and Rossdale share Apollo and two other sons, 11-year-old Kingston and 9-year-old Zuma. Rossdale posted a snapshot on his own account that shows him celebrating Apollo's birthday with his boys.
"HAPPY BIRTHDAY APOLLO. Pure magic boy," he wrote.
Stefani and Rossdale split in 2015 after 13 years of marriage. Stefani and her boyfriend, country star and The Voice coach Blake Shelton, dressed up as Daphne and Shaggy for Apollo's Scooby-Doo-themed party over the weekend.
Rossdale is dating Sophia Thomalla, and cozied up to the 28-year-old model in a new photo this month. He said in an interview with Loose Women in January that his sons are doing well but have yet to meet Thomalla.
"They're doing great," the star said of his boys.
"[They've] done a bit of FaceTime, just to kind of ease it in," he shared. "Gotta make sure it's steady from my perspective, as opposed to meet[ing] someone and you won't see them again."