A 52-year-old man with hypertension and benign prostatic hyperplasia (BPH) presents to the clinic. He has been on doxazosin 4 mg daily for 6 months with good blood pressure control (BP 128/82 mmHg) and improved urinary symptoms. However, he now complains of persistent orthostatic dizziness on standing, particularly in the morning, and occasional syncope during exertion. Vital signs show BP 145/88 mmHg supine and 118/70 mmHg standing (drop of 27 mmHg systolic). Heart rate is 88 bpm supine and 102 bpm standing. What is the most appropriate next step in management?