Are you sure your son has Congenital Adrenal Hyperplasia and not Adrenal Hypoplasia?  This would explain why both his 17OHP and his cortisol levels are low whilst off medication.  As already stated with CAH, 17OHP levels are generally high in the morning.  Although the treatment for both conditions is the same. in Hypoplasia, 17OHP levels are not raised as excess androgens are not produced.  Just a thought........ I assume you have had DNA testing?