Takaramono

From Ultimate Manga Guide:
Naoyuki, a sincere, very honest and direct country boy comes to Tokyo for high school and stays with his city cousin’s family. At first everybody takes advantage of him thinking that he’s too naive to catch on things, including his cousin, Satoshi who’s in college and demands that in exchange for tutoring him, Naoyuki have sex with him… Then Satoshi finds out that Naoyuki isn’t as gullible as he looks and starts falling in love with his pure-heartedness. Vol #2 introduces a new schoolmate of Naoyuki who also falls in love with him but Naoyuki is very confused about his own feelings for Satoshi and the difference between sex, love and friendship.
Sequel: Koibito no Jikan