Another thing. Do you have any ketone strips? If not you can buy them at any pharmacy or grocery store in the diabetes area. Run one of the strips under your dog's urine stream and see if he has ketones in his urine. If he does he needs to get into the vet's ASAP.
When is the last time that a blood glucose and fructose was run on him? As dogs age their need for insulin changes. He needs to be seen by the vet again.
Sounds like it's time to check his kidney function. You'll need to take him to your vet, and bring a urine sample along with you. If possible, get the first urine of the morning, and if you can't get it to your vet within about an hour, keep it in the refrigerator until you can. I used a plastic cup to get my dog's urine sample with no trouble. Just pop it under the stream and the dog has no clue. Don't wait on making that appointment! :-)