Josh posted a super sweet snap on Instagram of the bright-eyed seven-month-old gazing into the camera. Josh held him on his knee with a sign reading: "happy birthday Mommy!!"
Fans were quick to tell him how adorable the photo is, with several commenting how much the blond tot looks like Fergie.
The birthday girl was snapped on her big day going to the gym. Perhaps she was burning off some calories ahead of tucking into not one but two cakes given to her to celebrate turning 39.
One, from her friends, featured a topless picture of her hunky actor husband and pink roses made of icing.
Sharing a photo on Twitter, she wrote: "Thank u @BAkerlund, Veronica, @aamarirose & Marissa 4 my dope bday cake!!!!! LOL."
Baby Axl took a strring role on other cake, the centre-piece of which was a photo of the cute tot. Fergie was delighted with the sweat treat, writing: "My cake from my boyZ I love, Axl and @JoshDuhamel. #babycakez."
Also felicitating the musician on her 27 March birthday was her little sister Dana Ferguson. "Happy Birthday to the #bestbigsisterEVER! I love you Ferg!" wrote Dana, sharing a selection of adorable family photos of Fergie as a little girl.
This birthday is undeniably a special one for Fergie, being her first as a mummy. She and Josh have been on cloud nine since welcoming their "little man" last August, sharing several pictures of him.
"we're obsessed with him," Josh said late last year. "We can't stop staring at him. He's our little man, we made him together."
The birthday snaps show just how much their son has grown - the little lad is now sporting a lick of blond hair and he looks to be devolping green eyes, just like his mum's.