Last night he ate a little and was purring loudly when our daughter spent time with him. He's on a dextrose/potassium IV.
The vet tested for ketones when he was admitted and there are no signs of it.
We accidentally discovered what the issue may have been....we took over some spare syringes and the vet immediately noticed that the pharmacy had given my wife the wrong syringes, u100 not u40, so his insulin does have been way off for quite a while. An honest mistake but I'm pretty upset at Walgreens.
If he can continue to eat, he can hopefully restore his energy as the vet regulates his blood sugar. If anyone has any experience or advice, I'd be very grateful. Our vet seems excellent, a grad from Oklahoma Univeristy who has treated this condition several times before. So far the key seems to be to spend time with him and help him turn the corner.
Thanks to all for your thoughts, prayers, and advice.