A 4-year-old boy is brought to the pediatric clinic by his mother with concerns about delayed speech and social withdrawal. She reports that he began speaking single words at age 2.5 years but has not progressed beyond 10–15 words. He does not engage in pretend play, avoids eye contact, and becomes distressed when his daily routine is altered—he insists on eating the same breakfast every morning and arranges his toys in straight lines for hours. On examination, he shows repetitive hand-flapping movements and poor joint attention. His developmental milestones for gross and fine motor skills are within normal limits. What is the most likely diagnosis?