We had the same problem when they increased my sons cortef.  After many different doses, the endo. switched us to prednisone.  The weight  has leveled off.  He did not really "loose" it, he just kinda grew into it.  It is very hard at that age.,  That was exactly the same age that it happened to my son.  He is short, so he really didnt have much room to gain any weight.  But it will get better.