OK some good info! I'd still like to know if the vet said there were ketones if you have that information but with what you have so far given us:
6u PZI is a pretty high dose BUT if he is/has been on a dry food that may be part of the problem. Does he normally eat just dry?? I would venture an opinion that at home, he may be on way too high a dose because cats in a strange environment will "stress" sending blood sugar higher than it would be at home. I would not want to shoot that high of a dose, particularly on no eating. The only way I know for you to figure out really where you are at with dosage is to go out and buy a glucometer and learn to test at home. Some people do not want to do that, but it will be the only way to take out the guess work of what is going on. While you are there, I would look for ketone strips, which you can dip in urine to see if ketones are present. I know there are blood sugar urine testing strips but these will only tell you if there was a lot of sugar in the urine *hours ago* not what is happening now. Better than nothing but not that helpful for determining a present course of action. Most likely your vet did not address hometesting but it is more and more common, it CAN be done with regular human meters and getting a set of numbers over a period of 12 hours will be your best source of information on what is happening.
I would vigorously pursue getting him to eat. Get baby food. Get a force feeder syringe like I talked about earlier. Does he like canned tuna in water or canned chicken or deli chicken? Pick up one of those deli chickens for your dinner and see if the cat will eat it. I recently had great success with a friend's anorexic cat with bacon bits. It helped tremendously to get her eating wet food again, putting bits on top of the wet food. Is there anything he goes just nuts for? Get some and try to get him eating.
I hate to say anything about not giving insulin because it's not always best to give *no* insulin, but I sure would back off the dose especially with the limited food intake. The tricky thing about learning insulin is giving too much can 1) bring blood sugar dangerously low or 2) send it chronically high for a period of time before it comes crashing down low. EIther thing could be happening and you don't know without testing the blood sugar. (I know I sound like a broken record but that's just fact.)
I think you need to try to get him to eat before anything else. ANother thought is is he drinking? If he hasn't been again, syringe water. Dehydration makes you feel lousy and nauseated too.
I don't know what brand of PZI you have but most PZIs seem to work with a period of about 5 or 6 hours to the lowest blood sugar reading before it starts coming up again. It's not a huge fast drop, but I would be leery of dosing it on an empty stomach except maybe a very small amount.
Do you think you might be able to test his blood? There are some great videos here:
viewtopic.php?f=14&t=287
That would really help you figure out what to do. If you are at all comfortable with our advice, mine would be that for now, I would NOT shoot that amount of insulin. If you could get a BG (blood sugar) reading with a glucometer, it would tell us if you could consider shooting a small amount, which, if they are high, could maybe get him feeling well enough to eat better. Do you have anyone to help you? What city are you in? Maybe someone is near enough to help you learn to test.
I hope I am not overwhelming you but helping you to determine a course of action.