Aka to Kuro (KATSURA Komachi)

The story of Yukio and Yamato, who were born into yakuza families.

From Exiled Rebels Scans:
It was also raining the day I met Yamato Bidou. Yukio, the son of the second-in-command, learns from the boss’ son Yamato that his father doesn’t have long to live. “I don’t want your father’s, I want to exchange sakazuki with you, Yamato.” Yukio falls for Yamato, who is full of determination to take on the burden of the Bidou group despite being a high school student, and unofficially exchanges sakazuki with him. The two of them, both born into gangster households, are certain this is the meeting of a lifetime but…