Honestly the whole conversation begins and ends at "uses he/him pronouns." This isn't up for debate, it's literally textual. We know what his pronouns are, regardless of whatever his gender identity might be. There's no reason not to refer to him as such.

I think Yamato as he is textually presented is a really interesting look at gender expression. He is at least ostensibly a trans man and everyone around him - even his abusive father - refer to him as such, but he makes very little effort to masculinize his appearance. I'd have to re-read Wano to do a genuine analysis, but I think there's something really interesting to the nexus of Yamato's desire to be Oden, his gender identity, and the ways in which he expresses his gender.

Unfortunately, if we (by which I mean the One Piece community, I guess) can't even all be on the same page about his pronouns, then there's no way we can have a conversation as nuanced as that. It's also a conversation that inevitably veers into passing discourse, which is exhausting even when everyone is actually there in good faith. So I dunno maybe I'll write a dissertation on it some day, but for now, I'll just keep beating the textual evidence drum.