Grain-free food is best for a diabetic cat. Also, canned food is better for diabetic cats than dry, so canned grain-free is ideal. I leave dry grain-free food out for my diabetic cat, so he can eat when he is hungry if I'm not around, and then I feed canned grain-free whenever I'm around and he wants to eat. If I leave more canned food out than he will eat in one sitting, it just goes to waste because my cat will not eat 'left-over' canned food on the dish.
It's important to get your cat tested regularly when he is diabetic. The Vet can do blood-work to see if the cat's diabetes is in range. If the cat's glucose is too high, then he will need insulin shots. The Vets usually try to regulate diabetes with grain-free food first and if that doesn't work, then insulin is needed. Also, you can do blood tests at home with a home insulin monitoring kit by pricking the cat's ear for a drop of blood. Your Vet or Vet Tech can show you how. I have a diabetic cat and give him shots 2x/day. He has been diabetic for 3 yrs. (Note: Insulin is very expensive.) Good luck with your cat!