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If your cat hasn't eaten never give him insulin as that will only drive his blood sugar down further. If he's weak and can't walk you can rub some corn syrup or pancake syrup on his gums. The sugar will be absorbed through his mucous membranes. I'm assuming he ate fine and had his insulin this morning? If he feels much better then just don't give insulin tonight and get him to his veterinarian in the morning. If he still feels poorly go the emergency clinic tonight as hypoglycemia can be serious.
he has not eaten at all today and has been getting sick
he had his last insulin shot last night
It sounds like he needs help tonight then. I would get him to a veterinarian quickly. Good luck with Googles, let me know how things go.