Sorry to hear about your diagnosis. At least now you know what your dealing with. I have some experience with Cushing's so maybe I can help. I used to work at a vet so I am constantly "rehoming" dogs to my family and friends. About four years ago I rehomed an older, I think she was eight, Lhaso Apsa with my aunt and her family. Susie wasn't in the best condition medically and after doing bloodwork and other tests she was diagnosed with Cushing's. At first, it is alot of work to get the medication dosage correct. Basically, their is a feedback mechanism between the hypothalmus, pituitary,and adrenal gland. If either the hypothalmus or the pituitary become damged in any way they do not produce the hormones that are needed to induce the adrenal gland to function. After that the adrenal gland fails to produce the amount of corticosteroids the body needs to function properly. In order to manage this they follow a protocol that starts by killing the cells of the adrenal gland, usually done with lysodren (this way the adrenal gland produces zero corticosteroids). After that, you will give him a corticosteroid to replace the corticosteroids not being prouced by the adrenal glands. Then they go through a sort of trial and error period where they may change the dosage of medication he recieves based on his bloodwork results over that time. Once you get the dosage correct he should only need periodic bloodwork done to make sure the dosage is still correct, probably every six months or once a year.
Sorry this is so long and I know it seems daunting! Ecspecially, since your option is not a cure only management. The initial part is time consuming but after that the maintenance is not so bad. You might want to ask if he could have a mild sedative to help his anxiety before the office visit, I'm not sure if that would affect his blood work or not.
To make a long story short…Susie is now on maintenance for her cushing's and doing well. She isn't perfect but she is comfortable. In the long term, if you don't treat the disease you can expect to see more symptoms develop and the quality of life continue to go down. On the other hand, many dogs treated for the disease continue to live comfortable lives over the course of the next several years.
Good luck! Hope this info helped.