Tonari no Oniisan

From Shinmakoku Scanlations:

People say first love is the hardest to forget... And it is, at least for Hiroto. As much as he wants to, he just can't get Wataru out of his mind and heart. That guy next door, who is five years older than him, now even has a lover, and clearly is not interested in him. Hiroto has to learn to live with it... and with Wataru as his roommate, forgetting will certainly be the hardest thing to do.