I'm not sure what you mean by "Changed to diabetic good blood sugar decreased by 1."
I suspect that part of the reason that Lester's numbers are high is the dry DM. It's very high in carbohydrates. If you opt to switch him over to a low carb diet, please keep testing and transition slowly. Eliminating high carb food can drop his numbers.
Also there is nothing specific to treating diabetes in the Purina DM or any diabetic cat food, for that matter. If you read the ingredients, there is nothing that requires a prescription. In fact, the manufacturers have lost a class action suit for calling their foods "prescription." There are lots of canned foods that are lower in carbs. There are only 2 brands of dry food that are under 10% carb.