A 52-year-old man with CKD stage 4 (eGFR 28 mL/min/1.73m²) presents for routine follow-up. Serum calcium is 7.8 mg/dL (normal 8.5–10.5), phosphate 5.2 mg/dL (normal 2.5–4.5), and PTH 312 pg/mL (normal 15–65). Alkaline phosphatase is 78 U/L. He is asymptomatic. He is not on any phosphate binders or vitamin D supplements. What is the most appropriate next step in management?