胸なんてないですよ
男だと証明して欲しいというコメントをいただいたので
彼なりの男だという証明方法■背景は学校です（ちょっと特殊な学校）

He has no breasts.
He was told "I want proof that you're a boy."
This is his personal method of proving to others that he's a boy.
The background is at school. (It's a somewhat unique school.)