If your cat does not like prescription canned food you could offer a low carbohydrate one. Low carbohydrate canned foods that are nonprescription include Friskies Classic Pate Salmon, Fancy Feast - Savory Salmon Feast, Turkey and Giblets Feast, and White Meat Chicken Florentine. These are 2% or lower in carbohydrates. It is best to avoid any dry food because it stresses the pancreas. Be sure to offer sufficient purified water,not city water. Giving probiotics can aid digestion. Using Omega 3,6,9 can reduce inflammation and improve the coat.
The dose I use for DIA-IONX is 10 drops twice daily on the food or 20 drops once daily. I prefer to give it twice daily initially. You can use it along with insulin. You did not mention that. If you are giving insulin then you need to monitor blood sugar or the amount in the urine so that you would know if you need to lower the insulin dosage.
Please let us know how he does.