"I think he's going to be a great dad," Fatone, 37, told PEOPLE at the Disney on Ice presents Let's Celebrate! event in Los Angeles on Thursday. (Timberlake, 33, nor his wife, Jessica Biel, 32, have publicly acknowledged that they're expecting.)
Fatone says he got to see JT's parenting skills in action as the singer and actor looked after his younger stepbrother, Steven, and "took him under his wing."
Jeffrey Mayer / WireImage
"He's always been great with kids in general," Fatone said. "He's going to be great. I think he'll be very hands-on."
Timberlake is wrapping up his 20/20 Experience world tour on Jan. 2, and Fatone predicts his friend will "start slowing down" when he finishes: "He'll be taking some time off for sure."
Reporting by MATTHEW COLE WEISS