John Stamos recently marked a milestone.

On Saturday, the “Big Shot” star took to Instagram to celebrate the 3rd birthday of his son Billy, whom he shares with wife Caitlin McHugh.

“I can’t put into words the magnitude of the love I feel for this little guy. Happy Birthday Son,” he wrote in the caption, accompanying a montage of mirror selfies of father and son, with The Beach Boys’ classic “God Only Knows” playing as the soundtrack.

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“God only knows what I’d be without you,” Stamos wrote, adding the hashtag “#aftershowerseries.”

In a recent interview with ET, Stamos gushed about how the experience of fatherhood has changed him, and the life lessons he hopes to impart on Billy.

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“At the end of the day, can you look in the mirror and go, ‘I did my best. I did my very best?'” Stamos said. “I think those are the things I’m gonna be [teaching] to my son. Do your best.”