A 7-year-old boy is brought to the paediatric clinic by his mother with complaints of inattention and hyperactivity at school for the past 18 months. The teacher reports that he cannot sit still during class, frequently interrupts peers, and loses his school supplies. At home, he is impulsive, talks excessively, and has difficulty completing homework despite normal intelligence. Physical examination is unremarkable. The child's father has a history of similar behavioural problems in childhood. Psychometric testing shows normal IQ. What is the most appropriate first-line pharmacological intervention for this child?