We have a 14 yr old Norfolk terrier. He has had a hacking cough

Hello,
I am sorry to hear you are worried about Biscuit. I am a veterinarian (in the US) and will do my best to help.
I am sorry also to see you have not had help before now. I just came online and saw your question this morning.
Does he have symptoms of Cushing's disease, such as increased thirst and appetite, weight gain, thinning hair?
What antibiotic did he take? I usually treat a hacking cough with doxycycline and have found it to work after other antibitoics have been tried without helping the cough.
A chest Xray would be important to tell us what is going on. A hacking cough can be from an infection, heart disease, collapsing trachea, pulmonary hypertension, or pulmonary fibrosis. Some, but not all, of these can be diagnosed or suspected from a chest Xray.
I would certainly do a chest Xray before testing for Cushing's disease. Cushing's can be difficult and expensive to treat. Especially in older dogs with Cushing's I usually talk to the owner about the symptoms, testing and treatment before even starting with the first test.
There is a urine test, called UCCR, that is a good screening test for Cushing's. If it is negative, he has not got Cushing's. If positive, further blood tests are needed.
If you have not tried doxycycline, and if the Corvental is not helping, I would probably start there, and next do a chest Xray if he were my dog.
Let me know what else I can answer or help with. Sorry again I did not see this question (it came up at 2 AM my time!) earlier. I will be online for another hour or so.
Rebecca