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 the photo: "#NotJustanUncleAnymore #Overjoyed."

RELATED: John Stamos announces he's going to be a father for the first time at age 54

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."