Can't get numbers down! Time for new insulin?

Hello! New to the group. My boy Butter was diagnosed with diabetes on May 12th. Since then I have been giving him 2 units of Prozinc twice a day. After a few weeks I noticed he did not seem any better. I took him back to the vet and he increased his dosage to 3 units twice a day. I then began home testing. His numbers were middle to high 500's to high 400's. We are now 8 weeks into this and the vet has increased his dosage to 5 units twice a day. He does not look much better to me and we tested him last night and it read 480. He is now urinating on the carpet sometimes and still seems very hungry and thirsty. The vet recommended Hills W/D and low carb wet food which I am using FF Classics. He is eating about 2-3 cans a day. I give him half every few hours. I called the vet yesterday and he said that he is going to consult with a vet in Seattle before he raises the dose. He said sometimes the insulin needs to be changed. I am so upset and I just want my boy to feel better. All he does is lay on the kitchen floor and sleep unless he is eating or urinating. He's so skinny. He use to be a big boy. HELP!