Joan,
Unfortunately, your vet is right in a way. Cannisulin (called Vetsulin in the US) is a harsh, fast acting insulin in most cats. So the dry food may keep him safer from large drops because it will tend to keep his bg levels high. But it won't get you very far on the road to regulation or remission. Here is some info on Vetsulin:
viewtopic.php?f=19&t=302 As this guide suggests, Vetsulin is not being used much in the US, as it has had quality control problems. I am not sure if this is also the case in Australia; maybe someone else will know about Cannisulin overseas. Cannisulin was used successfully for dogs; it just doesn't work as well for cats. It tends to drop them fast and early in the cycle; then they tend to be high for another 6-12 hours. Other insulins that are less harsh generally give a lower drop over 6 hours or so and then a slower rise for the next 6 hours.
I know it is hard for you because you live so far away from the vet. In Canada, you can get insulin without a prescription. I wonder if that is the case in Australia? Someone posted in your earlier topic about someone else who lives there - maybe they can be a resource for you. Lantus, Levemir or PZI are the preferred insulins - they are milder and their users are successful in getting to remission and regulation. I know you can get Lantus in Australia. Another person posted here from the coast and is using it. She is down to tiny doses and doesn't post often but I will try to find her and have her contact you. (Watch the top right hand side of the page for "You have 1 private message".
And with this insulin, it is imperative that you start hometesting. You need to be aware of his levels if it takes him really low. Have you tried that yet?
I know it is hard to take advice from people on the internet over your vet. But if you read through the other posts, you see that our techniques are successful and safe. Everyone who posts here has or had a diabetic cat and was successful in treating them. Many of our cats went into remission; the other ones are well regulated, safely.