I mean the cosplay version of his brother is who he considers his first love and even thinks his feelings were returned at one point (his cosplay brother I think had unintentally told him that he loves him in french near the airport) so i think it makes sense
But even if he considers her as his first love, kids don't normally use this terms
It's usually used by lovers (like bloop bloop said) or by people when they talk to people youngers than them (for example, a grandmother/mother can call her grandson/son/daughter like this)
So for a French, it sounds a little bit weird/off but it's not a big deal ╮(￣▽￣)╭
You know how this kid is though. He is probably trying to act mature to woo the cosplay side of him (probably grew up seeing romantic movies with the male lead using those terms with their lovers )
it's sounds a little weird to me that he calls "her" mon chou since it means sweetie and it's usually used with your lover but well...
at the same time i can't think of smt else other than mademoiselle that could maybe sound better for his age and situation
i'm not complaining tho i'll definitely follow this manga ! (๑•ㅂ•)و✧