nico di angelo does not eat, as he thinks he deserves to be hungry (and he spent a portion of his life starving so he's used to it). he is working this out with his therapist, and is said to eat at the end of the book
A character with they/them pronouns is referred to as a "guy". They correct them and the other person apologizes. They are not referred to with wrong pronouns or gendered terms again.
A character does not eat a lot because they are used to feeling hungry. It is not mentioned in great detail outside of this chapter...
[MINOR SPOILER]
... and the character does get better and eventually eats more.