Have mercy! John Stamos‘ son Billy, 3, is growing up so fast, and the 58-year-old Full House star can’t believe it! John posted a slideshow of his only child through the years, as the little one prepares for his first day of school on Wednesday September 8. John’s “mini-me” couldn’t look happier to start school, even if his dad got a little choked up at how quickly his son is growing up!