Blue emails Simon that his Halloween was very quiet and he ate a bunch of candy. He admits that he's excited for homecoming, not because he likes football but because he loves the crisp fall air. Simon replies with a suggestion to just put a bowl of candy on the stoop next year and gives a brief account of the party he went to. He asks Blue if he ever feels locked into himself and like nobody knows his identity but him. Simon then brings up Spirit Week at school, which is a week of dress up days before homecoming.
Simon's question to Blue about feeling locked inside himself provides some explanation for why he's not necessarily able to look outside himself and empathize with others: he's too caught up in figuring out how to even exist in his own rapidly changing body. Spirit Week, like Halloween, is a chance for Simon and Blue to try on new identities for size with relatively few consequences.
Blue replies that he does indeed feel trapped into his identity, but in a way that's more about him holding himself back from doing things he wants to do because he's afraid. He declines to say what he's going to dress up as for Spirit Week, as he doesn't want Simon to figure out who he is. Blue doesn't think that their strange relationship will work if they know who they are in real life. In Simon's reply, he apologizes for making Blue uncomfortable and admits that he's nosy. He promises that he won't do anything to mess up what they have.
Blue's unwillingness to share his identity with Simon suggests that at this point, he's not ready to merge his gay identity (that at this point he shares only with Simon) with whatever identity he presents at school. Notice that Simon calls himself nosy; however, in doing so, he's talking more about one of his defining qualities than about a genuine care for other people.