My 4 year old daughter is also very muscular!  She has well defined thighs and her arms are so much stronger than kids her age!  She is very susceptible to the Cortef and showing side effects.  We have her on 7.5 mg per day spread out over 4 times per day.  Dosing this way we have been able to reduce her total dose and keep the side effects away (slow to no growth, puffy face and cranky and sleeplessness.)  What are your son’s levels?  OUr first endo was ok with her being off the chart low on all her levels.  This was definately not good for her.  Our current endo is more comfortable keeping her 170hp between 500-1000.  Good luck with your son.  (I know it is all so unsure I feel like it’s a guessing game sometimes especially when she is sick!)  Julia