Are you testing his BG at home. If you aren’t, then you can’t even know how well the insulin is or is not working. You can’t know how low the dose is taking the cat, and that can be dangerous, especially with increasingly large doses. I have helped many people with cats who start out in extremely high numbers (500s, 400s or who never get lower than 300s) to get them much healthier and some even go into remission. Watching for “clinical signs” like how much water they are drinking pr how much they urinate isn’t a good way of helping them. That is a very outdated way of caring for a diabetic cat and was done back before human glucometers for monitoring blood glucose were invented (~late 1970s and early 1980s.) Having curves done at the vet also only gives you a one-day picture of the numbers. If a cat happens to have higher blood glucose that day because of vet stress or other factors that I can explain if you want me to, then the vet may think that a dose increase is needed when it may not be. Anyway, I am just saying that what is really important is to know how low a particular dose of insulin is taking the cat. Then you can keep your cat safe and make dose adjustments based on that information. How have you been monitoring these things so far? It’s all really overwhelming at first, I understand.
One really important question is: what are you currently feeding your cat. I hope you have already transitioned him/her to a low carb wet food diet, which is essential for diabetic cats. A high carb or dry food diet will keep the blood sugar high.