A 3-year-old boy is brought to the developmental paediatrics clinic by his mother with concerns about delayed speech and social withdrawal. She reports that he spoke a few words at 18 months but has regressed and now uses only single words inconsistently. He does not make eye contact, avoids physical contact, and spends hours lining up toy cars in repetitive patterns. He becomes distressed if his routine is altered. On examination, he shows poor joint attention and does not respond to his name. He has normal hearing on audiometry and normal developmental milestones for gross motor skills. What is the most likely diagnosis?