Lyn what kind of insulin do they have him on?
There are some good kinds available now though the newer (better) ones tend to be more expensive. As Lynne says, it may be worth considering changing the type of insulin. (Some cats respond better to some types than others). Vets are sometimes unwilling to try experimenting but if he's not stable then one has to look at what might get him there.
With a blood glucose so high it's no wonder he's still drinking and weeing lots. He's probably not feeling great either (though clearly better than he was).
You could also try (with vet's permission, but I'd argue quite hard for this and consider doing it anyway) increasing his dose every week or two weeks by a very small amount and seeing how he does. (I mean by half a unit or even less if you have a syringe you can read that accurately). You're the frontline nurse in this instance and you're the one who'll know best and most immediately how he's doing. If you try this gradual increase for a month then he's blood-tested again you'll find out whether you might have reached the right level.
When I first had a diabetic cat I had just moved house and had signed on with a vet I discovered wasn't particularly good (I soon changed). I did this gradual increase without asking them, as their approach was so cautious we'd never have got him stabilised. It worked and I'm glad I did it. The dose we ended up on was 3 units (100%) higher than the one recommended by the initial vet.
Is he on one dose a day or two? And do you notice differences throughout the day?
Also I'd highly recommend feeding him high protein and very low carbohydrate food. This is going to take some research (as most commercial cat food has more carbohydrate in than even an ordinary cat needs let alone a diabetic), or help from your vet (there are now some high protein special diabetic foods on the through-vet market as science has realised that diabetic cats are not- as was previously thought- like dogs or humans, and therefore need a different diet). Certainly he shouldn't eat anything like Whiskas, Felix, Brekkies, etc. which have sugar in (a real no-no). You might even try one meal a day of plain-boiled shredded chicken breast- but buy organic and/or one with no added sugar... yes, even some shop-bought packaged plain chicken breast can have sugar added! Time this meal according to the type of insulin he's on and when that's least effective in relation to his meal times... If you adjust his diet at a very early stage then you may find you don't need to increase the insulin so far to get him stable. Which would be a good thing for several reasons...
