The missing kid in Stand By Me is Ray Brower.
The film follows four young boys, Gordie Lachance, Chris Chambers, Teddy Duchamp, and Vern Tessio, as they embark on a journey to find the body of Ray Brower, a boy who went missing a few weeks earlier.
The boys' adventure takes them through the woods and into the heart of a mystery surrounding Ray's disappearance. While the film doesn't explicitly reveal what happened to Ray, it suggests that he was accidentally killed by a train.
The story of Ray Brower serves as a catalyst for the boys' journey of self-discovery and friendship.