A 13-year-old boy has developed concentration problems in school, accompanied by a significant decline in grades. When evaluated, he explains that he has been worrying about his grandfather, with whom he is extremely close. His grandfather was recently diagnosed with Alzheimer's disease, and the boy has been helping his mother take care of him. As his grades decline, he becomes increasingly sad and his energy level drops, which further hinders his ability to pay attention in school. He also complains of being restless and unable to sleep. What is the most likely diagnosis?