A 5-year-old boy from rural Bihar presents with short stature, bowing of legs, frontal bossing, and rachitic rosary. Serum calcium is 7.8 mg/dL, phosphate 3.2 mg/dL, alkaline phosphatase 420 IU/L, and 25-hydroxyvitamin D is 8 ng/mL. PTH is markedly elevated at 180 pg/mL. X-rays show metaphyseal widening, loss of sharp metaphyseal margins, and ground-glass osteopenia. The child has no history of malabsorption or renal disease. What is the most appropriate immediate next step in management?