Have you tried freeze dried chicken? You can get the dog version. If he likes that, you can sprinkle some of the dust on some wet food to entice him to eat. You can also start with mediums carb wet food to get him to eat it. Here is
Dr. Lisa's list of canned foods with nutritional content. "Low carb" is less than 10%, medium carb is 10 to 15%, and high carb is above 15%. Make a list of some foods to try and get a couple of cans. Even today, Zener won't eat low carb food so we give him medium carb. It's less than ideal but a whole lot better than dry food.
Liz
ETA: I think Dr. Lisa evaluates dry food too but they are all high carb. Plus they are dry, which is bad for all cats and especially bad for diabetic cats. They need more water in their food for their kidneys to work well. Smokey, eat that canned food!
For your mom, could you find a different vet that is more on board with feeding a better diet? Or have mom post here? Vets are amazing with treating so many different animals and different diseases but they don't know feline diabetes like the folks here.