HI MummyGeek,
Have your Father get his Cortisol Level Checked. More Than once. AM Fasting is best. If low, he needs to see an Endocrinologist to have an ACTH STimulation Test (also called an Cortrysin Stimulation test) to see if he has Adrenal Insufficiency. That's what long term steroid usage does - Kills the Adrenal Gland. It was done to me. At 13 I began getting Kenalog shots once or twice a year. When that no longer helped the Dr's prescribed MASSIVE doses of Prednisone, Medrol, Dexamethasone, whatever to get me back breathing again.
Now at 43 - I was diagnosed at 40 - I have Secondary Adrenal Insufficiency.
It is a SERIOUS disease that left untreated Can KILL! If he has it he needs to get on treatment immediately. For some reason Doc's DON't like to diagnose this disease - I think because by doing so they pretty much admit that Medical Science has let us down & CAUSED this Disorder. You'd never be able to PROVE it however. Too many variables.
Without having more information about your father it is impossible to make any connection from his current symptoms and his previous therapy. I would suggest speaking with his physician.