Get to your vet ASAP! If you've already been there, as I imagine you probably have, and your vet isn't concerned, find another vet, at least for a second opinion. If there is a veterinary school at a university nearby, that's a good place to look. If not, there may be a vet who actually specializes in handling the more serious diseases that your vet might refer you to. If your vet isn't willing to help you, again, find another vet! You might be able to do some searches on the internet to find someone appropriate near you.
By the way, I assume your vet has advised you on how to adjust your dog's diet for his diabetes?