Aisaretagari no Surface

Itsuki Hiruma is cast in the role of the "brother" of Kei Kasugai, a younger popular actor he has always admired. They end up living together during the filming period to get into character, but Kei—who was believed to be a polite and well-behaved young man—turns out to be a playboy who sleeps with anyone he meets! When Itsuki tries to be brotherlike and scolds Kei, Kei knocks him over asking the older actor to "rehabilitate" him with his body!

(Source: Tokyo Mangasha, translated)