Dear Loving Grandmother,
You should be commended for stepping in to care for your grandchild during this time of need. As you know, it ain't easy.
As to whether his moods and appetite are effected by Prednisone, the answer is -- maybe. The only way to tell is to get his doctor to try another medication and see how he responds. There is A LOT of trial and error with this disease. Prednisone is strong and fast-acting, but I was on it as a child from age 6-16, then went to hydrocortisone for 20+ years.
How do you deal with his moods & tantrums? Same way you would any other child. While I have often wondered if I could "blame" certain issues I've had on CAH or my medication, the answers always include simply working hard on the problems. As a parent or grandparent, you must be compassionate, but firm.
I wish you all the best --
Sincerely, Eric B