A 7-year-old boy is brought to the child psychiatry clinic by his mother with complaints of inattention and poor academic performance. The teacher reports that he frequently interrupts classmates, fidgets constantly in class, and struggles to follow instructions. At home, he loses toys, forgets to complete homework, and is unable to sit through meals. His mother reports he was a hyperactive infant and has always been "on the go." Physical examination is unremarkable. His IQ is average. Which of the following is the most appropriate first-line pharmacological intervention for this child?