Saikyou Pashiri

Ryunosuke Takagi, once known as the "Invincible Dragon" and the strongest brawler, is now a bedridden old man living with his grandson, Kento. Kento, who is bullied and forced to run errands for delinquents, struggles daily with his unfortunate circumstances. However, one day, an unexpected tragedy and miracle strike simultaneously, changing everything!

(Source: Nihonbungeisha, translated)