I'm sorry Riley was diagnosed with diabetes. I, myself, had a diabetic boy, several years ago.
A diabetic cat's diet should be high in protein and low in carbohydrates. Also, it is beneficial for Riley to eat a wet food diet, rather than dry. He can graze on a small, measured amount of dry food, but the major portion of his diet should be wet/canned. Stay away from foods that are more than 10% carbohydrate. Read labels, so you're familiar with the ingredient panel and percentages of carbs and other nutrients. Also, stay away from canned food with gravy--this provides non-nutritional calories for Riley. Canned Fancy Feast and Friskies Classic Patés are usually good for diabetic cats. Other foods you can buy in pet supply stores and are grain-free (the best type) are Wellness, Wellness CORE, EVO and Felidae.
The following site is written by a feline veterinarian and gives detailed advice on how best to feed a diabetic cat:
I hope this helps and all will be well with Riley!
Please feel free to ask any other related questions and I will be online for about another hour.
Please take a moment to rate with positive feedback (5 **Stars** is the best).