A 14-year-old boy presents to his pediatrician with concern about bilateral, tender breast enlargement that he first noticed 4 months ago. His physical exam is notable for Tanner stage 3 pubic hair, a testicular volume of 8 mL, and a palpable, concentric, fibroglandular mass under each areola. The remainder of his history and physical exam is unremarkable. What is the most appropriate next step in management?