Zero no Sekai de Hana to Odoru

Abandoned in front of Viscount Irino's mansion, Asahi grew up with Touri, the son of the viscount, like a brother. Touri, who treated him like a real brother, was the center of the young Asahi's world. As time went by, Touri grew up to be a beautiful and respectable young man, and he was as friendly to Asahi as he had been when he was a child. One day, Touri attended a party of a noble family and came back with an aphrodisiac. Asahi knows that he is not the right person to touch him, but he can't resist his desire to do so...

(Source: Tokyomangasha, translated)

Note: Includes one extra chapter.