A 5-year-old boy is brought to the clinic by his mother because he says he has been talking to his recently deceased grandfather. The grandfather lived with the family, frequently cared for the child, and died 2 months ago. The boy was kept out of preschool for 3 days after the death but has since returned and is reportedly doing well; his teachers say he completes his assignments and plays appropriately with his classmates. During the visit, he becomes tearful and says, “I miss my grandpa. I sometimes talk to him when my mom is not around.” He has no changes in sleep, appetite, or behavior, and there are no academic or social difficulties. Which of the following is the most likely diagnosis?