A 4-year-old boy from rural Uttar Pradesh presents with bowing of legs, delayed walking, and recurrent respiratory infections. His mother reports that he spends most time indoors and eats a diet poor in milk and fortified foods. On examination, he has frontal bossing, rachitic rosary, and genu varum with a waddling gait. Serum calcium is 6.8 mg/dL, phosphate 2.5 mg/dL, alkaline phosphatase 280 IU/L, and 25-hydroxyvitamin D is 8 ng/mL. X-ray of the wrist shows widened metaphyseal plates with cupping and fraying. What is the most appropriate initial management?