I started taking Kuvan almost a year and a half ago. When I started my dietitian said the only way we would be certain Kuvan was causing my levels to drop would be to consistently eat the same amount of phe. This means your son will need to be strict about what he eats to be sure that he is eating about the same amount of phe each day. If his levels consistently stay the same like they normally would if someone off of Kuvan were to eat the same amount of phe then Kuvan is not working. If his levels drop despite the fact that he has not made any dietary changes then Kuvan is probably working. I hope this helps!