Lovely Sick Manga Summary
From DramaQueen:
A lover's possessiveness can make the beloved feel extra special. For high school student, Naoyuki Akiyoshi, that special brand of loving is given 24/7 by Ryouichi Sumi, who is not only his lover, but is also his primary doctor and legal guardian.
As Naoyuki matures under Sumi sensei's care, he begins to push against the protective boundaries that Sumi sensei has created. Sometimes, change can lead to a deeper bond. And a new beginning.