It may be the amount of Cortef.  Are his levels very low?  Is he oversuppressed?  It may be the timing of the Cortef.  Is he on 2 or 3 times a day with each dose rather large?  Or is he on 4 or 5 times a day with each dose small?  These things can effect weight gain!!!!  I would look into it.  Some people are very sensitive to side effects of Cortef and do better on smaller doses spread out in closer intervals.  My daughter is on doses at 3am, 7am, noon, 5pm and 10pm.  The largest dose is at 3am with each dose getting smaller until the smallest at 10pm.  Good luck  Julia