A 6-month-old infant with confirmed classic phenylketonuria (PKU) is brought to the metabolic clinic. Plasma phenylalanine is 850 µmol/L (target <360 µmol/L on diet), and the mother reports poor adherence to the phenylalanine-restricted diet due to difficulty obtaining medical foods. The infant has normal development so far, but you are concerned about long-term neurological outcomes. What is the most appropriate immediate next step?