I don't know much about Hyperphe as I have Classic PKU, however I would like to say that even if your boy does end up with a diet it is unlikely to be anywhere near as restrictive as the Classic PKU diet so please don't panic.
The other thing is if he ends up on diet it is for the good of his health, it can be a scary prospect but many, many parents have to go through putting their child on a diet, whether it be for Celiax, Allergies, Lactose Intolerance or diabetes, I remember my mother saying once that she has talked to parents of children with other disabilities and they wish that the severity of their childs disability could be reduced or even stopped with something as simple as a diet. Basically we are very lucky for our diets and it is not as bad as it seems.
If you do have to limit your childs protien your medical team will work with you and help you through it, but for now relax and accept that the doctors believe that he may not need a diet, take the rest as it comes, it doesn't help worrying what might be, cos it may not happen, but if it does I believe that you will have the strength and determination to do what is right, what mother wouldn't want to do the best for her child?
If you have any other worries I would be glad to help, I can only imagine how scary this is for you, but take it from someone with PKU, it truthfully isn't that bad, and we are fortunate to have websites like this where we can request help or hear about others experience.