The button?

I believe the second button on a school uniform is the closest one to the heart, therefore a symbol of love when graduation comes around (cuz you don’t need it anymore), so basically Yamaguma gave his heart to him.

Thank you so much! That makes so much sense now..

yas I’ve already read so much mangas that I learned this part of their culture that is so sweet and unforgettable. soo kawaii ヾ(❀╹◡╹)ﾉ~