A 4-year-old boy from Delhi presents with bowing of legs, delayed tooth eruption, and recurrent respiratory infections. His mother reports he spends most time indoors and has a diet primarily of refined grains with minimal milk. On examination, he has frontal bossing, rachitic rosary, and genu varum. Serum calcium is 7.2 mg/dL, phosphate 3.8 mg/dL, alkaline phosphatase 420 U/L, and 25-hydroxyvitamin D is 8 ng/mL. X-ray of the wrist shows widening of the growth plate with metaphyseal cupping and fraying. Which of the following is the most appropriate initial management?