He may not be on enough cortisol to stabilize his adrenal glands. He seems to be getting plenty of salt.  I would consult with another pediatric endocrinologist. Where are you? Is there a children’s teaching clinic in your state? You may be safer going there as they are up to date on research and knowledge. Again, when my daughter went back into the hospital after she was born, the ped endo increased her hydrocortisone dose greatly. I felt it was too much medicine for a tiny baby, but she said that’s how they find the dose to stabilize on.  They slowly lower this high dose until the levels are stable. Keep trying and never give up. My prayers to your son.  with love,