We had a diabetic basenji some years ago. She had lost weight and then suddenly got very sick, spilled sugar, and had elevated blood glucose. We started out with one insulin shot everymorning, she protested at first but I got very quick and pretty soon she hardly noticed. Back then, my vet treated by symptoms…if she started drinking a lot, I increased the insulin. Periodically we would check her blood sugar every 2 hours throughout a day to see how efffective the insulin was. After a couple of years we went to 2 shots a day, still not a big issue. She was about 12 when we started, so I don't know how she might have done if she had been younger, but we got another couple of great years for her before she lost sight and then got a bit senile at about 15.
That was about 12 years ago, treatments may have improved a lot since then. And there are great 'low carb' foods like EVO that should help a lot! We have freinds whose Great Dane became diabetic, very nearly died, and she started feeding him cooked ground beef, no grains at all, and he improved and lived another couple of years, I think he was about 12 when he died and had been diabetic for several years. Diabetes treatment in humans has improved a lot over the years, I'm sure it is the same for dogs. I would get him on a low-carb food right away, and see if that helps.
Good luck, hopefully with good diet control and exercise, he will do very well.
Anne in Tampa