The actor and 31-year-old McHugh married in February as his publicist says "Billy and Mom are both home and healthy". The couple named their son Billy, after Stamos' father.
"From now on, the best part of me will always be my wife and my son", he captioned the Instagram, adding the hashtags #NotJustanUncleAnymore and #Overjoyed". They broke their pregnancy news back in December.
Caitlin, 32, went on to describe the look on her partner's face upon learning of their lovely baby news.
"You know how you put your father on a pedestal, and he's a superhero, and then as you grow older, you go, "Ah, he's just a man"?"
"The look on John's face when I told him we were pregnant was priceless", McHugh told the outlet. He always wanted to be a dad, but at age 54, he felt that ship had sailed.
Congratulations, John and Caitlin! "She's so magical", he said of his new bride. "I've been practicing for a long time".