From my personal experience, the 24 hour urine and saliva tests were the most important, and blood tests were always off, since they are really just a quick snapshot in time at any given moment. Overall symptoms are what really need to be considered. For 4 years I was told I was "normal" based on AM fasting Cortisol blood serum levels, until the urine and saliva tests over 24 hours showed definitively just how screwed up my Cortisol levels were.
But to me it was the long term symptoms that were the thing really needed to diagnose. You need to look over many things and keep track over a period of days, weeks and even months. Are his Circadian Rhythms off (he is overly tired in the evening and then wakes up at 10 or 11PM, can't fall asleep or stay asleep until 1 or even 3AM, then has difficulty getting going and staying awake all day)? Is his skin thin, so that he cuts or bruises easily? Is he anxious during the day, and feels like he has a hole in his stomach no matter how much he eats. Does he crave carbs and sugar (since cellular energy levels are low) and salt (since water regulation in the body is off due to adrenal issues)? Does he have allergies or chemical sensitivities, or does his immune system seem low? Does he fatigue easily and his muscles have trouble recovering (muscle aches and pains for way too long after exertion)? Does his blood pressure vary dramatically, with it being too high at times and too low at others? Is he puffy, or have trouble with swelling in the face, feet, ankles, hands or abdomen that comes and goes? Is his mouth excessively dry, especially at night (generally caused by higher than normal blood glucose levels)? Is he experiencing night sweats for no known reason? Are his blood glucose levels slightly elevated or low over time? Is he subject to panic attacks and easily stressed out (this seems to be caused by electrolyte and sodium imbalances caused by faulty adrenal regulation due to too much or too little Cortisol)? Is he gaining or loosing weight dramatically?
You can monitor most of these things yourself at home and keep a diary if the answer to too many of the items is Yes.
For me, as a man, the big one was that excess Cortisol excretion eats up Testosterone levels, which then fall dramatically. Though I was diagnosed as an adult, I would assume that as a young boy or as he begins to enter puberty, his testosterone levels should be fairly stable and standard for a boy his age, and I believe they even soar as he becomes a young teen. If his levels are too low, even if still in the "normal" ranges, together with slightly elevated Cortisol levels, then look to Cyclical or Episodic Cushings. Sorry to say that that's only the beginning of the journey, because then you must continue looking for the cause of the Cushings, if indeed that's what it is.
There are so many variables when it is hormonally related and very elusive when it is Cushings. Ultimately though, Cushings is rare and Adrenal dysfunction in children is also rare, although not unheard of I'm sure. But it's a bit early to panic based on a short period when he seems to be "soaring" considering his age and the growth spurts that occur in children. It could just be such a spurt, or that he is entering puberty early and his hormones are raging so to speak, with all of this being perfectly normal. If you notice more of the Cushings symptoms you can find in this forum and watch for them over longer periods of time coming and going, together or not, with either higher or lower than normal Cortisol urine or saliva testing and lower than normal Testosterone and/or Thyroid levels, something other than natural aging may be going on.
Good luck and hope you find a good Pediatrician who is open minded, steadfast and doesn't jump to conclusions and medicate.