So, considering the mangaka's other, canned manga, I have some guesses.
He is FTM, either getting the disease by lucky chance, or perhaps even doing so on purpose. I think this because of the name change and how his mom still sees him. Also, the way he was talking to the MC makes it clear he is very well versed in gender and identity, which could be just from experience of having the disease first, sure, but more likely because of already working out gender and looking up things.
However, considering his reaction in the shower, I will assume that he does not have a community or people to talk about this. His mom sees him as a tragic girl, while no one else seems to know meaning he switched schools and/or moved. With how the MC never asked for this, and has it without choice, that hope for a connection was shattered, and he has locked himself in the closet since he cannot tell the MC.
The scars are likely from not being accepted, be that pre or post disease. He could have been trying to transition beforehand, only to be bullied by his father, or likely, the girls in his class. I am guessing that he always liked sports growing up, and played with boys, but the girls got jelly of the friendships and bullied him pre transition.