Well, I don't know how severe your cat has the disease, but he might live a
fairly long time without suffering if you control his diet.....I have
diabetes myself, and I find that if I watch what I eat, I can keep my blood
sugar under control without insulin shots......I would put down just water
for your cat, and give him small amounts of food throughout the day, so that
he doesn't eat a lot at any one sitting. Getting blood from him for a blood
sugar test would be a real PIA with a cat, so I would try to collect his
urine and test that instead.....They make test strips that will show you how
much sugar he is passing in his urine by changing color. Cats don't seem to
like starchy foods anyway, so if you feed him good stuff, like chicken and
fish, he might live a long time......

We had a cat when I was growing up that had diabetes. He didn't seem suffer
at all, and he was around 10 or 11 when he died.

He got insulin twice a day, and we checked his urine with dip sticks you can
buy. He even learned to urinate in an empty litter box so it was easier to
check, just make sure it's fresh urine.

The only thing our vet told us to keep on hand was some of the clear corn
syrup in case he crashed. Which is no different than a human diabetic
needing sugar if their sugar level gets too low. Luckily we never needed to
use it.

As far as food he was eating c/d cat food since he had problems with his
urinary tract being blocked before he became diabetic.