This was a book that an elementary school teacher read aloud to us (mid 90s), and she deliberately kept the title a secret so that we would have to come up with one on our own. What I remember of the book is that the boy lived with his community (family, friends) on rafts on a river that is entirely in a cave system. Importantly, they never leave the caves (I think the boy hypothesizes at one point that the people used to go into the caves for the winter, but lost their way out and have forgotten the exterior world over time?). The boy falls off the raft one night while everyone else is sleeping and loses his family because the rafts keep floating down the river. He eventually finds a hole to climb out of the cave and is stunned by the sunshine. After that, I don't remember any more of the story.