"Bill states that the average at 9 months 90% growth."
kev,
that is an average. it does not mean everyone will experience 90% growth. also, just because all of his hair may have grown does not mean it's fully thickened and matured.
i am speaking from experience here, whereas you are speaking from 6 months growth of your first ht. plus, you are already a bit panicked yourself. my point is, should he be concerned--of course! it's only natural to feel that way. hell, i'm concerned and i'm only at 4 months. but the reality is that you cannot come to a full conclusion until 12 months time. every single ht doc will tell you that as will many experienced posters here.
p.s. spex has posted a few times an example of a patient and his thickening between 8-9-12-18 months. the difference is dramatic.