A 2-month-old neonate born to non-consanguineous parents presents with poor feeding, irritability, and lethargy on day 5 of life. The infant was normal at birth. Serum ammonia is 320 µmol/L (normal <50). Plasma amino acid profile shows elevated glutamine, alanine, and lysine, with low citrulline and arginine levels. Urine orotic acid is normal. Genetic testing confirms homozygous mutation in the CPS1 gene. Which of the following is the most appropriate initial management?