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Definitely a good idea to get your kitty started on a canned high protein, low carb diet for diabetes. It works best if combined with insulin injections--that will give him (and you) the best chance of getting him into remission and off insulin.
I assume your kitty is a little overweight? For a typical 10 pound cat, I would shoot for about 150-180 calories per day. If you are using Fancy Feast (or similar-sized cans), there is about 90 calories per can. So 1 - 3 oz can twice a day would be perfect (and timed well with the insulin injections!). For larger cans, the 5.5 oz, usually 1 can per day is adequate. But every food has a different calorie content, as well as protein and carb content.
Here is a really good website that lists pretty much all canned foods available and compares the protein and carb amounts: http://catinfo.org/