I have a character who is revealed as being trans some way into a series, but it's a non-transphobic society so it's not played as a shocking reveal, just as something that someone new to town didn't know before. I couldn't figure out earlier how to tip off the reader earlier, since he's not a POV character and the other characters have just thought of him as male ever since he first said, "I'm a boy."

I'm not at all fishing for an "Oh, that's fine then!" response! I'm just thinking about your last paragraph. I don't know how it will actually come across to readers, but I think in context what it's most revelatory of is the culture of the town.