A 3-year-old boy is brought to the pediatric clinic by his mother with concerns about delayed speech and social withdrawal. She reports that he rarely makes eye contact, does not respond to his name, and prefers to play alone. On examination, he is observed lining up toy cars in a repetitive manner for extended periods and becomes distressed when the arrangement is disturbed. He shows no interest in interactive games and does not point to objects of interest. Developmental screening confirms significant delays in language and social reciprocity. What is the most likely diagnosis?