A 25-year-old man with a known diagnosis of hypertrophic cardiomyopathy (HCM) presents for routine evaluation. He reports one episode of near-syncope during exertion two weeks ago. His father died suddenly at age 42. He is asymptomatic otherwise. Physical examination is unremarkable aside from a harsh systolic murmur at the left sternal border that increases with Valsalva. Transthoracic echocardiography reveals a maximum interventricular septal thickness of 34 mm with a resting left ventricular outflow tract gradient of 20 mm Hg. A 24-hour Holter monitor shows occasional premature ventricular contractions but no sustained arrhythmias. Which of the following is the most appropriate next step in management to reduce his risk of sudden cardiac death?