The couple split almost a year ago but it seems the couple are on good terms, reuniting for the party, which was held at a private house.
Proud mum Fergie shared a sweet snap from the day, where she dressed up as Wonder Woman.
“happy bday party day to my favorite superhero #axljack,” she captioned the pic.
Dad Josh also took to Instagram, sharing a video of the day, complete with ice-cream truck delivery and a superhero battle in the backyard.
Fergie and Josh announced their split in September 2017 after eight years of marriage, and Josh recently opened up about how it works co-parenting Axl.
"We have a great relationship, we really do. She's a great mother," the Transformers star told E! News earlier this year. "All we want is the best for our son. So it's a bit of a juggling act but we're both grownups who can handle a lot. And he's happy, that's the main thing."