I think you might want to consider taking Lucky to a specialist. He is older and something like pancreatitis could potentially kill him...it can a young dog as well; but he is vulnerable.
Four vet visits sounds like a bit to make one diagnosis....when my vet is not sure, I run to the specialists.
If your vet cannot refer you to one, you can do a search at:
American College of Veterinary Internal Medicine > Home
Please let us know how Lucky is doing!