Primary is adrenal - secondary is pituitary - he has a pituitary issue as well? If so, that all has to be watched and he needs a good pituitary neuro-endo.
I am so sorry to hear about your son. I know steroids are important for some things, but sadly it is overlooked about the impact on some - some people can handle the load but others drop into AI, others into Cushing's.
I have had both Cushing's and now I am AI as my adrenals were removed to resolve the Cushing's.
As for the pain, yes, there is pain. Cortisol is an anti-inflammatory. So when there is a wide swing is cortisol, the body is not happy with the loss of cortisol. When I went from high to low (I was cyclical) it was very painful. There are some that say it is akin to a heroin withdrawal but I have never done that so I don't know. Sorry, warm baths is just not enough. He needs cortisol - it may be that he could just get a tiny bit just to tide him over. I am hoping your doc gives you extra, and you cut up pills. I get 10mg pills and carry around cut up pills so try giving 1/4 a pill and see if that helps. Or salt, or something salty.
Is he on regular replacement? When and how is he replaced? How is he stressed dosed? How do you give him extra salt? Sodium is a critical part of low cortisol and that can help a lot in keeping cortisol normalized. In fact, I take another medication to help retain sodium - fludrocortisone. Is your son on that med? How are his sodium and potassium levels? It can be in some that sodium is good but you need to avoid high levels of potassium like bananas if he is ill (but I am odd, I need potassium as I drop in both so I need both when sick so you have to know tests).
Do you have an emergency set up for him - a medic alert tag, an injectable medication and needle, instructions and extra meds, and something to prevent vomiting? For us, loss of fluids (vomiting or runs) is dangerous, so it has to be stopped or we need fluids in the hospital ASAP.
As for long term, it is more like management... making sure people around him are aware so that if he gets sick he gets correct treatment right away. Also when he plays he needs fluids and salt (we people without adrenals are loving the nuun tablets as low sugar but high salt) more than normal people. He needs a salty diet (I carry salt with me) like pickles and olives and stuff - when I get sick bouillon cubes are my friend.
What replacement is he on - hydrocortisone, prednisone or dex? HC is the norm as can imitate a normal body...