A 3-year-old boy from Delhi presents with recurrent episodes of vomiting, lethargy, and developmental delay. Serum ammonia is 180 µmol/L (normal <50). Plasma amino acid profile shows elevated glutamine and alanine. Urine orotic acid is markedly elevated. Genetic testing confirms ornithine transcarbamylase (OTC) deficiency. What is the most appropriate immediate management?