"HAPPY BIRTHDAY DADDY!!!!" wrote the Black Eyed Peas singer alongside the photo on Twitter, which was promptly retweeted by proud dad Josh.
Baby Axl, who is just 10-weeks-old, was clearly excited by his dad's 41st birthday celebrations and is seen beaming at the camera.
First-time parent Josh recently spoke about how and he and Fergie, 38, have fallen in love with their 'little man'.
"First of all, we're obsessed with him," Josh told E!. "We can't stop staring at him. He's our little man, we made him together."
"I think there's something you can't really describe until you go through it," he added. "I think any parents out there would understand that. It's just an incredible time for both of us."
Fergie and Josh, who have been together since 2004 and married in 2009, have shared several pictures of Axl.
"Axl Jack Duhamel Day 1," posted Josh nearly a month after Axl was born on 29 August. "#paparazziyoucanleaveourhousenow."
"Gonna catch some football with my little guy today," wrote the actor by another cute photo of himself cradling baby Axl.