A 72-year-old man with benign prostatic hyperplasia and a prostate volume of 55 mL on ultrasound presents with persistent lower urinary tract symptoms despite 3 months of tamsulosin monotherapy. His PSA is 6.2 ng/mL and post-void residual remains 95 mL. He is normotensive and has no cardiac comorbidities. What is the most appropriate next pharmacological step?