HI! My name is carrie. I'm a mamma of 3- and my youngest has classic pku (the most severe form) and has been treated from day 7.
I want to tell you that you really don't have anything to worry about in terms of your son's development. I've researched pku extensively in terms of research available in canada/usa and europe as well and those with hyper phe simply don't have the cognitive issues that those with classical pku (even treated) have to deal with.
What is most important for your son is consistency. Apparently higher numbers for hyper phe causes fewer issues than high fluctuating numbers – ideal fluctuations keep within 100 points/week.
In terms of your questions.
We in canada do the same numbers we follow a 120-360 scale for classical pku- and up to 600 for hyper phe.
If you have hyper phe it doesn't change- he might have a slight variation in his numbers over the first year or so but it won't change to classical pku (requires a different genetic mutation)
Results can change day by day and show up within a day . We get tested once a week – more when she's really struggling.