You could also see it as a kid trying to act mature so they don't seem/feel out of place. He does, indeed, have the actual mentality of a kid at the very minimum right now, despite knowing a lot and being smart. It was also shown he feels a lot comfier in kid form, but that if my memory is right...
I can faintly remember he was quite childish even when in adult form as well, even if only lowkey... Like, did things or said things only a kid would suggest... Also, could be remembering wrong, but Abigail even commented on this during pre-kid-Verite chapters, I think... o3o
My current theory is that he's the fairy prince who either got tricked or messed up and ended up stuck in the mirror.