Maybe the belly button is just part of the midriff fetish as a whole and he's not familiar with the term?
And really, with so many women exposing their bellies these days, it's as if the fetish has become normalized and not have as much cultural value as others do. Strange times we live in.
Saying the belly button fetish is just part of the midriff fetish is like saying the armpit fetish is just part of the (side)boobs fetish, or feet fetish being part of the legs fetish. The adjacency is what contributes to its being, but it's also not exactly the same thing.
It's not really normal nor is it common. Maybe you mean it to be tame and not disgusting? It's actually weird. On the contrary, exposure makes a body part less likely to become a fetish. In a place where midriffs aren't normally exposed, I've heard of a guy getting
too excited at the sight of a tiny sliver of bare waist in the gap between the blouse and the skirt.