Slake's Limbo, by Felice Holman, 1974

Aremis Slake, a neglected and ill-used foster child, had learned how to dodge conflict by slipping into the subway system at the first sign of a threat. One day he slipped in and stayed. His first brushes with order, dignity, and compassion were found underground, where he slept on newspapers in a cave he made his home, and shared his food with a rat. 33 years after publication the novel still feels remarkably current, despite an unusual, omniscient voice; unsentimentalized, candid realism that offers Slake convincing victories and hope. Strongly recommended.