A 3-week-old male neonate born to non-consanguineous parents presents with poor feeding, irritability, and a musty/mousy odour on the nappy. Newborn screening by tandem mass spectrometry shows elevated plasma phenylalanine (>20 mg/dL) and elevated urinary phenylketones. The infant has no developmental delay or seizures yet. What is the most appropriate immediate next step in management?