Saenai Oji-san to Ore-sama Yankee-kun

Subaru, a short-tempered young man, is walking one day when he casually saves a dull-looking man from trouble. The older man, two decades his senior, smiles and tells Subaru he’s a good boy. For the first time in his life, Subaru is smitten! Even after they part, Subaru can’t stop thinking about him... is this really happening?!

(Source: Nebula Club, translated)