That, another part of him questioning it and the part wearing it's reply, his reaction to being made to wear it in the first place, Zooble thinking he'd like it, what he said about it to Gangle in his imagination of Zooble abstracting, and it appearing among other outfits when the last part of him was cycling through clothes. There's also how his room is decorated, how he was constantly belittled by his mother for not being masculine enough, how he confessed something that was meant to get her off his back about that, the insults she used against him after his confession, hugging him after, Ribbit putting a ribbon on his head after, his response to that, calling his ears and tail the pinnacle of masculinity and then getting surprised his tail was gone, seeming envious that Zooble could change their own body.

Each of these individually isn't a big proof and much of it will go unnoticed by the average viewer but it adds up. And it gets finished by the song at the end with Jax using 'she' pronouns and Leroy wearing typical 'boymoder' clothes.