I had an outie as a kid - it turned "innie" as I grew - around the teen years, as I "filled out" it stayed where it was, and the rest of me grew around it. Now it is a definite innie. My daughter has an outie too, you can see it sticking out through t-shirts. We are going to let it go until she says it is an issue - if she ever does....then I will tell her how mine changed as my body changed, and ask her to wait it out. Hers is not herniated.
We did have friends who had a little girl with a really big outie and hers was herniated, and they had surgery at about 4 years old to fix the hernia - and that helped it not stick out - now hers is about flat.
Just a thought, the herniated ones are covered by insurance I think - but anything else would probably be considered "cosmetic surgery" and you would have to pay out of pocket. You might just want to wait and see how it changes as he grows, and talk to him about the fact that everyone is a little different in many ways - skin color, eye color, belly button types, fingerprints, and explain that it just is part of who he is.
Good Luck whatever you decide.
Jessie