Just have registered at this site and saw your post. I have the same: old dog (Dusja) with diabetes and totally blind. I sent a Blind Dog Story and the admin promised to publish it shortly on the site. Reverting to your subject: unfortunately, I don't know the reason why your dog has lost an appetite, but for sure something is wrong. Because diabetic dogs have extremely good appetite. My dog can eat now 24 hours a day, and she was not like that before. When she was diagnosed diabetes, doctor has not explained very well all care procedures. So I was giving Dusja the insulin shots and feed her in about 2-3 hours later. Now I know the brake can be max. 20-30 min., if shot is made in a thigh. This neglectance caused a diabetic coma, and only being under system for several days helped to save her life. She was very weak, could hardly stand and did not want to eat at all. A vet. inserted a catheter in her vein and gave me glucose, which I was pouring through catheter regularly. Vet. told me, if she will not start eating soon, she is going to die. We managed to overcome all these troubles and doing more or less fine.
Why I am writing all this to you? I have 2 suggestions for you:
1. Find another vet., who can determine the reason of appetite absense.
Those obviously could not.
2. If the problem has not been solved yet. you can meanwhile give him the glucose through the same method, so that your dog will not be fallen into diabetic coma, like mine did. He needs to receive food after insulin shots!
I wish you good luck and soonest recovery to your dog.
If I can be of help with any other advice, please write. Will be gald to help.