Cat With Diabetes - At my wits end..

[Miaow]

Our approx. 9 year old male neutered cat was diagnosed with diabetes last summer. When diagnosed his blood was in the mid 20's. He has fairly severe neuropathy. He was started on Caninsulin and after increasing several units, he was getting big peaks and valleys in his readings but always stayed in the high teens/20's.
Our vet suggested we switch him to another type of insulin made specifically for cats that is longer lasting and supposed to reduce the peaks and valley's. It is called Pro-zinc. It has been a long struggle. We had him up to 5 units, and he was staying approx. between 15-20 during the day. We increased again to 5.5 units and now he started having huge fluxuations from the high teens down to 3-4 and back up. We went back to 5 and his numbers stay again between 15-20. Tried upping him again, and same thing huge changes. This is all over the course of many weeks and with the consultation with our vet. She is telling us to stay at 5 units. He is now getting very constipated despite being on a type of "laxative" and had to go to the vet today and he was straining really bad. Nothing against our vet, but she doesn't seem to have too many suggestions and reasoning why the huge fluxuations and I wonder how much experience she has treating feline diabetes.
We are really at our wits end and hate to see him struggle walking, getting in and out of the litter box and just collapsing on the floor exhausted. I should mention he has been on a Methyl-B12 supplement for a couple weeks but from what I read it is useless until his blood is normal.
Does anyone have any suggestions? I don't want him to suffer any longer without seeing some type of improvement.