A 72-year-old man on tamsulosin 0.4 mg daily for BPH presents after 8 months of therapy with persistent moderate LUTS (IPSS 20). He reports nocturia ×3 per night, weak stream, and incomplete emptying. DRE shows a large, smooth prostate (estimated >50 g). PSA is 3.2 ng/mL. Post-void residual is 120 mL. Renal function is normal. He has not experienced acute urinary retention. What is the most appropriate next step in management?