Have mercy! John Stamos, Caitlin McHugh welcome baby boy

Uncle Jesse is officially a father, and John Stamos' adorable announcement has us saying, "Have mercy!"

The 54-year-old Stamos posted a photo his son's arrival to Instagram, saying, "From now on, the best part of me will always be my wife and my son. Welcome Billy Stamos."

He tagged it: "#NotJustanUncleAnymore #Overjoyed."

Billy is named after John's father.

In December, the Full House and Fuller House star announced that he and fiancee Caitlin McHugh were expecting a child in an interview with People Magazine.

Stamos, who is a first-time father, said he was ready because he's had "years of practice."

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