A 6-year-old boy with confirmed autism spectrum disorder (ASD) is referred to the child psychiatry clinic by his school. Teachers report that he has become increasingly aggressive toward peers during unstructured play, hits himself when frustrated, and has difficulty transitioning between activities. His parents confirm similar behaviors at home, with episodes lasting 10–15 minutes and occurring 3–4 times daily. He has no seizure history, normal hearing, and no known medical comorbidities. His IQ is estimated at 65 (moderate intellectual disability). Which of the following is the most appropriate first-line pharmacological intervention for his behavioral dyscontrol?