I was under the impression that what type of PKU someone has is based on what their phe levels are when they are not following the low protein diet. Because doctors no longer take people with PKU off of the diet, which type of PKU someone has is partially based on phe levels from the Newborn Screening.
If blood phe levels are greater than 20 mg/dl (1200 ummol/L) when off diet, then the person most likely have classical PKU. Phe levels between 10-20 mg/dl (600-1200 ummol/L) indicates atypical or mild PKU, and phe levels between 4-10 mg/dl (240-600 ummol/L) indicates hyperphe.
Of course to be 100% positive which type of PKU your child has you would need to have his mutation tested.