ET's Lauren Zima spoke with the 56-year-old actor on Monday on a red carpet celebrating The Little Mermaid Live!, where he discussed his boy's first words while discussing preparing for his role in the TV event. He plays Chef Louis, the eccentric French cook who tries to put Sebastian the crab (played by Shaggy) on the menu. However, it gets a little difficult when Billy feels chatty.
"I'm rehearsing my number, kinda for the first time on stage -- jumping on tables with knives and things -- and all I could hear was 'Dada, Dada, Dada, Dada!'" John said of his son's interruptions.
When asked what else Billy has been saying, he playfully stated, "Trust fund were his first two words… He does talk. He says, 'coco.' He wakes up in the morning saying 'coco' and then he says… Lilo is his dog. Lilo, Lilo! Coco, coco! Mama, mama!"
John also gushed about getting to play Louis, a role that he says his wife, Caitlin McHugh Stamos -- a serious Disney fan -- loves that he's taking on.
"I love it," John shared. "I'm not a great singer, so to play a character… I feel sorta protected in this part 'cause, like I said, it's a big character and there's a lot of dancers and you have the Disney magic behind it all so.. I just hope I don't fall."
Also joining the Full House alum in the production are Auli'i Cravalho as Ariel, Queen Latifah as Ursula and Graham Phillips as Prince Eric.
Last month, Caitlin shared with ET that she wanted her and John to move out of L.A. before having their son in April of last year.
"I decided, let's wait until we move -- which we recently did," Caitlin said. "We were living in John's bachelor pad, basically, that he's been in for 13 years and it's a beautiful house…[but] it wasn't conducive to raising a family."
"We're all about community now, we moved into Hidden Hills… We're settling in the house in the new area and now feels like the time to really try [to have another child]," she added. "We're not [pregnant] yet, but we are trying. We'll see, hopefully in the new year we'll be able to add to our family."
When discussing how many kids they want, Caitlin said, "I'd be happy with one more. My arms can only grab so many children at once. I might get a little stressed out with more than two. I don't know, we'll see!"
The Wonderful World of Disney presents The Little Mermaid Live! airs Tuesday, Nov. 5 on ABC.