I know that the Metabolic Nutritionists in the US are working on creating standards across the clinics. At our clinic, we are told that levels should be between 120 and 360. We aim to keep Molly, who is 10 and has CPKU, between 120 and 240.
The system we use for counting protein/phe is different from yours. We count mg of phe and pretty much everything Molly eats — including all fruits and veggies. It is a pain but gives us a better sense of exactly how much phe she is getting (Again, she has CPKU so she likely needs tighter control).
Is your son getting enough protein? Molly gets over 65 gm from her formula and another 8 gm from food. We were always told that since the protein from the metabolic drink is synthetic and so broken down, the body cannot utilize it as efficiently as a more natural whole protein. As a result, some of it is lost in the urine before the body can use it. Because of this, our kids actually need more protein than the USDA standards recommend. If the body is not getting enough protein (or calories), it is pretty resourceul and will become catabolic, breaking down muscle in the body to get what it needs. Since this muscle is high in protein, the levels go higher.
Sometimes if our kids do not get enough calories and protein their phe levels rise and it appears that their daily phe/protein allowance is lower than it actually is.