The Boy Who Turned to Ash

(from ebookjapan):
George Akiyama's theater of terror, a masterpiece of suspense from a boy's point of view! Ushio is a boy from a happy family, but one day, a strange man whispers something to him that makes him uneasy, and he begins to doubt his father and mother. One after another, his anxiety and suspicions grow, and eventually he discovers the horrifying truth...? There is a secret that her father and mother are keeping from him...?

An early work of Weekly Shonen Jump.

Note: Bunko (also available digitally) collects it with part of Akiyama's seinen horror thriller Fish Lage.