BAT Kyuuketsuki Tantei Oliver Sunshine

Oliver, a kind-hearted nerd attending high school in New York, is involved in a mysterious incident one day, leaving his mother seriously injured. Miraculously, he recovers, but something feels wrong with his body. Later, at a church he visits, Oliver is told he has become a vampire.

Confused but desperate to find a way to save his mother, Oliver is entrusted to a suspicious private detective named Yan. However, Yan is a vampire-specializing case-solver who doesn't shy away from violence and discipline! As a half-baked vampire, Oliver is untrained and sometimes finds his life in danger, but he finds himself working with Yan to investigate cases involving monsters…?

(Source: Kadokawa, translated)