How old is your cat? Did he have full screening blood work done?
What was the high sugar level on Friday? Did he have sugar in his urine?
Has he had an xray?
Thanks for that information. I agree with you that a second opinion should be obtained for your cat. I would recommend trying a cats only practice, if one is nearby. Here is a link to the American Association of Feline Practitioners. You can search for a cat vet in your area:
The high blood sugar is a concern. Cats can have a stress elevation of blood sugar, but 400 is exceptional high. Diabetes is the first thing to rule out. A urine sample should be obtained by your new vet and analyzed, the checked for sugar. Another blood sugar should also be checked at the same time. He may be diabetic, or he may have pancreatitis, an inflammation/infection of the pancreas. The new vet may also pick up something on exam that the other vet did not.
The rapid weight loss is also a concern. He may benefit from a feeding tube for a short time, until he gets through this. As you probably know, rapid weight loss for any reason in cats can cause a fatty liver disease, hepatic lipidosis. Hopefully that hasn't occurred yet, and we want to keep that from happening.
I hope this was helpful. Please feel free to ask any follow up questions you have. I would be happy to talk further with you.
You're doing a great job with your boy at home. It's hard to feed a cat entirely by eyedropper, but if you can aim for about 80-100 ml (ccs) in 24 hours, you're doing pretty well. That plus the Pedialyte will keep him hydrated and keep some calories going in him. I hope they can help him tomorrow. Make sure a urine sample is obtained.