A prey-model raw diet seems to make the most sense to me as meat contains no starches... thus keeping the blood sugar from rising.
At the very least, I'd go with a hi-protein, low carb kibble such as Orijen, EVO, NV Instinct, etc.
Taste of the Wild may be a good choice since it is grain free, but lower in fat than EVO or Orijen.
Please keep in mind that I am not a vet and I have never had a diabetic dog, so I am only using my common sense here.
However.. I don't have much faith in most vets' nutrition recommendations. I would not feed Hills, Iams, Royal Canin and that other crap that vets recommend. It makes no sense to me to "heal" the body with poisons such as BHA and BHT... not to mention the other extremely questionable ingredients.