A 4-year-old boy from rural Uttar Pradesh presents with bowing of the legs, delayed tooth eruption, and recurrent respiratory infections. His mother reports he has been exclusively breastfed without any vitamin D supplementation or sun exposure. On examination, he has frontal bossing, Harrison's groove, and rachitic rosary. Laboratory tests show: serum calcium 6.8 mg/dL, phosphate 3.2 mg/dL, alkaline phosphatase 420 IU/L, 25-OH vitamin D 8 ng/mL, and PTH 240 pg/mL. X-ray of the wrist shows widening and cupping of the growth plate with metaphyseal lucency. What is the most appropriate initial management?