Tell him that it is an awesome thing to be different from the other kids, even if it's something like his belly button. Tell him that not being the same as everyone else is what makes him special (but don't say "special" in that tone that might imply slowness).
Hey, I wish I had an outie, so then mine would be a belly BUTTON and not a belly tunnel XD XD
But whatever you decide to do, good luck, and assure him that outies are definitely cute!
Dark Stranger