I'm sorry to hear about your situation. Unfortunately there is unlikely to be a simple fix for a 13 year old dog that has Cushings and now is not eating, vomits and lies around. Potential causes for the vomiting and weakness include complications from untreated Cushing's disease such as renal failure, kidney infection, pancreatitis, and diabetic ketoacidosis. Some of these diseases are more treatable than others, but the key to treatment would be early diagnosis with bloodwork/urinalysis and aggressive treatment with intravenous fluids for dehydration and treatment of the underlying issue. If further workup and treatment are not an option, then humane euthanasia would be preferable to letting him due from dehydration and starvation. The average duration of survival of dogs with Cushings disease is about 2-3 years post diagnosis, so it is likely "his time".
Kimberly Coyner, DVM DACVD