Though at first glance you may think that the snap is Williams' son Otto and a family friend, it turns out that it's Spencer and his older brother Michael.
Spenny posted the picture to share some news about his documentary, Finding Michael, where he set himself the task of trying to find the body of his older brother who died while scaling Mount Everest.
Some people who had already watched the project were commenting on how moving it was while others couldn't get over how much Otto looked like his father at the same age.
"Otto looks just like you in that photo," one person wrote.
Another added: "It was an amazing film. Otto is the split of you," and a third added, "Otto is a mini-me of you."
Others seemed to think that the youngest of the Williams-Matthews household looked more like Spencer's older brother Michael.
"Your youngest boy looks so much like Michael," commented one individual.
Spencer teamed up with Bear Grylls and Nirmal 'Nims' Purja, a highly experienced mountaineer, set off on their mission to try and recover Michael's body.
Michael became the youngest British person to ever reach the summit of the peak at the tender age of 22 but three hours after the incredible achievement he disappeared.
