The snap showed the pair smiling together and clearly in the party mood with Dec holding an unbelievable balloon creation, while David towered over him. In his caption, David joked: "Me & my young son. I can't believe he just turned nine. @antanddec," and while Dec celebrates his birthday in September, David's son, Alfred, marked his ninth birthday last Friday.
WATCH: David Walliams speaks about his son
Fans found the snap and David's fake out hilarious, with many sharing strings of crying with laughter emojis, including close friends Denise van Outen and Natalie Imbruglia.
One teased: "Omg, they grow up so fast!" while another jested: "He's doing so well!" and a third commented: "Where is your other son Ant???"
A fourth posted: "Aaahahahaha this made me howl!" while a fifth added: "What a charming young man aged nine."
David joked about his 'young son'
David shares his son with ex-wife Lara Stone, and the pair are very private with their son, with neither having shared any photos of the young boy.
David enjoys these fake-out captions, and often refers to his dogs, Bert and Ernie as his "one true love" or his "sons" and sometimes captions nights out with his mum as a "date night".
However, last month he enjoyed a night out with close friend David Schwimmer and he kept the caption straight, instead writing: ""Dinner with Schwimmer @_schwim_."
The star often refers to his pet dogs as his "sons"
In the photo, the men grinned from ear-to-ear in the snap with David wrapping his arm around the US star, who sweetly rested his arm over David's shoulders.
Fans rushed to share their disbelief over the photo and compliment the men on their dapper appearances. "Two incredible talented gentlemen. What a gorgeous photo. Handsome two Hot Daves," replied one.
A second said: "WOW! Special moment. A third added: "This is absolutely so much love," and a fourth said: "No way! My two favourite people."
Like this story? Sign up to our HELLO! newsletters to get other stories like this delivered straight to your inbox.