Milk Carton 1986

When Kenny McCormick, a smalltown boy, suddenly goes missing and is nowhere to be seen, his older brother attempts to find him. With nothing to go by but a shaky handwritten note and Kenny's picture printed on a milk carton, he doesn't even know if he can find him alone. Did Kenny simply run away, or is there something darker going on? (Darkfic, NOT a oneshot. Kidnapping, etc.)