Phe levels are suppose to be between 2-6mg/dl or in your case 120-360umol/L. Milligrams per decaliter and umol/L are just two different ways of measuring phe levels. This means because your sons levels have been between 270 and 330 they are exactly where they need to be! When he gets older and starts to eat solid foods or if his levels get higher then his doctor or dietitian may start restricting how much phe he takes in to keep his levels between 120 and 360. As of right now they are probably not worried about restricting the amount of baby formula or breastmilk he is getting. Because his levels are stable and where they should be whatever amount he is getting must be the right amount.