NTA - I, personally, would let him know beforehand. Although I'm currently single-ish, I like informing any partner of mine of stuff like that. That's my personal choice.

Does that mean you should feel like you need to discuss that with him? No. You have every right to modify your body to whatever extent you like, regardless of what your partner thinks, as long as no one else gets hurt (although I'm not sure what body modification might cause harm to others).

But I'd also like to mention that he doesn't have to like it, and if it's enough of a dealbreaker for him he has every right to leave. And if someone is going to throw a hissy fit and dump you over a belly button piercing, then I think that just shows what kind of person they are.