A 14-year-old male with ADHD combined presentation has been taking mixed amphetamine salts extended-release 25 mg daily for 10 weeks with excellent symptom control, his grades have improved and his teachers report significantly better focus and behavioral regulation. However, his mother is concerned because he has lost 4 pounds (from 112 lb to 108 lb), frequently skips lunch at school, and reports not feeling hungry until 7-8 PM. His BMI has dropped from the 45th to the 35th percentile. He denies intentional food restriction and has no history of disordered eating. Which of the following is the most appropriate initial management approach?