Firstly when you say Cortisone do you mean cortisone acetate or hydrocortisone?
Secondly why doesn't his doctor try Prednisolone in the morning as well as in the afternoon/evening first, before thinking of changing to Dex. The early (the earlier the better) morning dose for any CAH person is very important.
Ideally the biggest dose of the day (and the most potent - in his case Pred) should be taken in the early hours of the morning about 4am to cater for the early morning cortisol surge, which in a 'normal' person takes place at that time. However in a CAH person if little or no steroid replacement is on board at that time it only succeeds in causing the ACTH to rise further thus pushing out excessive androgens. People aren't often all that keen on taking a dose at that time of morning so compliance can be an issue, that is why doctors often suggest taking a large dose last thing at night before bed. This is not ideal. We are not meant to have high cortisol levels in our blood stream during the early hours of the night when sleeping.
CAH can be challenging in the teen years to control and this could explain why your son is less controlled at present.
This is a long link but it shows the normal cortisol pulsatile rises in a 24 hour period.
http://images.google.com.au/imgres?imgurl=http://www.becomehealthynow.com/images/organs/endocrine/circadian_rhythm.jpg&imgrefurl=http://www.becomehealthynow.com/popups/circadian_rhythm.htm&h=294&w=496&sz=24&hl=en&start=3&tbnid=xG6YvRvZC0FPaM:&tbnh=77&tbnw=130&prev=/images%3Fq%3DCortisol%2BCircadian%2BRhythm%26svnum%3D10%26hl%3Den%26safe%3Doff%26sa%3DG
Also this is interesting - not a medical journal article but written by an MD. http://www.nutrition4health.org/NOHAnews/NNW02HardingAging.htm
