A 3-year-old boy is brought to the developmental pediatrics clinic by his parents with concerns about delayed speech and social withdrawal. The child was developing normally until 18 months of age, when his parents noticed he stopped responding to his name, lost interest in playing with other children, and began repeating words without understanding their meaning. On examination, he makes poor eye contact, shows hand flapping when excited, and lines up toys in a row rather than engaging in pretend play. He has no significant medical history and normal hearing on audiometry. What is the most likely diagnosis?