Halloween // Need Help With Belly Button Costume

So we just had some friends over for dinner and their 3 y/o son told us he wants to be a belly button for Halloween. His parents have no idea where he came up with this idea and kind of shrugged it off. My partner and I want to make it a reality as it would be hilarious. He has a super goofy personality so he could totally pull it off. Any good ideas on how to make this happen? We thought about making a big round paper mache and having his face be the button?? Appreciate any help!