When Remus came out as non-binary in fifth year they never corrected people who used the wrong pronouns. That was always Sirius.
Slughorn was the worst for mis-gendering Remus.
'Remus my boy!' Etc.
And Sirius always saw Remus bring their arms a bit closer in and try and make themselves seem a bit smaller. They hated attention so they never corrected Slughorn. It was Sirius that did that.
'Professor Slughorn, Remus is they not he'
And Slughorn would always nod and smile and apologise. And do the same thing next lesson.
Dumbledore had been the one to ask Remus if they wanted to move dorms as they weren't male. But Remus declined, saying they would rather be with their friends. Gender identity didn't change friendship.
Lily was, of course, the first to know. Remus always went to Lily for advice about anything. After all she was the one who introduced them to biromanticism and asexuality. During prefect duty they had been talking together and Remus explained that they 'don't feel like either gender and am I even more broken?'
Lily explained about non-binary and Remus smiled because there are other people out there who feel the way they do.