A 58-year-old man with chronic kidney disease stage 3b (eGFR 35 mL/min/1.73 m²) and osteoporosis (T-score −2.7 on DEXA) presents for management. He has had no prior fractures. His serum calcium is 8.2 mg/dL (normal 8.5–10.5), phosphate is 4.8 mg/dL (normal 2.5–4.5), and PTH is 120 pg/mL (normal 15–65). What is the most appropriate next step?