Small dog not eating well?

My dog has gone from 16 lbs to 12. He had cushings disease but seems to be okay with that. The count is okay when he was last checked. Specialist noticed he had lost weight and took blood test, didn't see anything. Sent him to regular vet, and he really didn't see anything either except for a little arthritis so gave him a med for that. That doesn't help. My little dog just eats very minimum and I am afraid he is getting weaker. He is an 11 year old JR. Any suggestions?

[Jaybay]

Did your vets specifically look at his kidney function?  If he is in the early stages of renal failure, it may not have shown up on the blood and tests.  That's true for about any condition.  Are there any other symptoms that would explain the weight loss?  Loss of appetite, vomiting?  Any change in urination frequency or bowel changes?  How about activity levels?  Does he still show interest in his usual things like toys or even barking at something outside?  All these things will help your vet with a diagnosis, so watch carefully and maybe even keep a diary of symptoms.