Once Sara finds her brother, why won't he go with her?

In the novel, Sara's brother Charlie wanders away and becomes lost. Sara later finds him in a ravine, which he will not leave because his watch has stopped working. Charlie, a mentally challenged, ten year old, places great importance on his watch. He correlates being lost with the fact that his watch isn't working. For him, the fact his watch isn't working is directly associated with the fact he doesn't know where he is.