A 7-year-old boy is brought by his parents for psychiatric evaluation due to persistent refusal to attend school over the past 3 months. His mother reports that each school morning he cries, clings to her, and complains of stomachaches that resolve once he is told he can stay home. He sleeps in his parents' bed every night, refuses to attend playdates without a parent present, and recently became extremely distressed during an overnight at his grandmother's house, calling home repeatedly and begging to be picked up. He has nightmares about his parents being in a car accident and not coming home. He has no academic difficulties, no reported bullying, no social conflicts with peers, and his teachers report he is well-liked when he does attend. He was previously a good student who enjoyed school prior to these symptoms developing. What is the MOST likely diagnosis?