Soshite Bokura wa Koi o Suru

From Dangerous Pleasure:

Tomoya Kurata, having no living relatives, has just lost the elderly Kaname Tsukiyama to whom he was particularly close. One day, while grieving alone in the old-fashioned house where the memories of her remain, he encounters Hikaru Tatsui, who declares he is Kaname's grandson. "Would you mind living here with me?" he abruptly proposes, and Tomoya is forced into living in the same house. At first Tomoya is disgruntled, but is the feeling of being unable to turn his back on the somehow slightly lonely Hikaru gradually turning to love?