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 was born via normal vaginal delivery with no perinatal complications. By 18 months, he had spoken 10–15 words, but since then his language has plateaued and he has lost interest in playing with other children. On examination, he makes minimal eye contact, does not respond to his name, and engages in repetitive hand-flapping movements. He lines up toy cars in a row and becomes distressed if the arrangement is disturbed. Developmental screening using the M-CHAT (Modified Checklist for Autism in Toddlers) is positive. Which of the following is the most appropriate next step in management?