It’s seems strange that your Endo would not have seen anything like that before, unless your son’s levels were abnormally high.  Make sure you follow up with him on your most recent test.  He sounds like my endo, I had to call him a few times and remind him before he gave us his test results back.

In our case my son has an 11-hydroxylase deficiency rather than the more common 17-hydroxylase deficiency.  This deficiency is less likely to be a salt wasting issue but to be sure we had numerous sodium and potassium tests done.  Has your endo been sending you for these? It would seem to me (I am not an expert) that if his levels are still high that he would need more medication.  At our most recent 9 month c/up his levels were tested and came back normal but if they were not his dosage would have had to be adjusted.