Non-PKU HPA is the same as hyperphenylalaninemia or hyperphe for short. In simple terms hyperphe is a mild form of PKU. Classical PKU is the most severe case. It is hard to tell if your son will need a low protein diet or not, but in my experience many people that I have met who have hyperphe still need to follow the low protein diet to some extent.
The main difference between those who have classical PKU and those who have hyperphe is that people with hyperphe tend to tolerate more phenylalanine (the amino acid that our bodies do not break down) without having high phenylalanine blood levels. Only time will tell how much phenylalanine/protein your son will be able to have. For the most part treating PKU is a lot of guessing and checking. You do a blood test and if the phenylalanine level does not come back where it needs to be his doctor or dietitian will adjust his phenylalanine intake to bring his levels back to where they need to be.
I can promise you that with proper treatment your son will grow up to be an amazing man! I have classical PKU, and I just see PKU as a different way of living. As long as I follow the low protein diet and drink my formula nothing can stop me from doing what I want to do.