Thank you for your question. My name is***** I have been a small animal veterinarian for 14 years and I'd like to help with your question.
The canisulin is one of the best so I would stick with it. Some dogs just need a high dose and the dose you are on now sounds like only a medium dose for a lab. We define insulin resistance as a dose of 1unit per pound.. so for a 50 pound dog, you could go all the way to 50U twice a day before you completely give up on it.
What is probably going on in your case is that the cushing disease is causing insulin resistance. Is his cushings treated and well controlled? Dogs with uncontrolled cushings have too much cortisol in their system and that can make them resistant to the insulin so you keep raising the dose and doing glucose curves, but you don't really get any where.
If it isn't already controlled, you should focus on treating and controlling the cushings.... then the diabetes will likely come into control also.
I hope that helps, but if you have more questions please let me know.