Feed him then follow the protocol given by your vet.
As soon as possible, administer honey or corn syrup by rubbing it on the gums (even if unconscious, but not if in seizures), and rush it to the vet. Carry more honey or corn syrup with you on the way and keep rubbing it on the gums, where it can be absorbed -- it could save the pet's life. Every minute without blood sugar causes brain damage. Some recommend administering syrup anally, with a feeding syringe or dropper, if the animal is in seizures!
NEVER try to make an unconsicous animal swallow. The food or liquid could possibly choke him/her. There is also a chance that the materials could be aspirated (wind up in the lungs instead of being swallowed).
Intervet suggests an especially good place to also rub sugar, syrup or honey is under the pet's tongue.
If the pet has hypoglycemia according to the blood glucose meter (<2.2mmol/L or 40mg/dL), but no symptoms, give treats or food if possible. If they won't eat, try putting food in their mouth.
If that doesn't work, administer some honey or syrup followed by food or cat treats, and continue to do so until the blood glucose is rising, and the latest insulin shot's peak action is past.
The honey, syrup or "fast-acting" sugar will make the blood glucose rise, but the rise will not last very long.
Feeding something with carbohydrates in it when the pet is able to take it will make sure hypoglycemia doesn't overtake the "quick" sugar fix.
According to BD Diabetes Diabetes in Pets website, the amount of syrup for dogs is: for small dogs-one teaspoon; for larger dogs- one tablespoon. For cats, they opt for one tablespoon of syrup for every 10lb. in body weight.
Intervet suggests 1 gram of glucose for every kilogram (2.2 lb) of the animal's body weight and extra food every 1-2 hours to counter the insulin's effects.
If the symptoms do not abate within 15 minutes, follow up with another tablespoon of corn syrup. If there's still lethargy and/or sleepiness after a total of 2 tablespoons of corn syrup are given, contact your vet right away for more information.