A cat with too low of insulin levels will wander and appear lost. There may be a glazed look to his eyes and even a cloudy appearance. The cat may seem off balance or fall over. If you feel this is the case then you can rub some karo syrup on his gums to see if he responds positively. Insulin shock usually only occurs in diabetic cats.
Did you try the Karo syrup?
How old is he?
Has his urination and thirst increased more than usual?
Was there other blood work or urine tests run yesterday?
When did he last receive his insulin and how is his appetite?
If he gets insulin twice a day then I would withhold the second dose due to not eating. Try offering him canned food, tuna, boiled chicken, even jarred baby food in beef, lamb, chicken, or turkey. This will not only help with hydration but will also keep his glucose more stable. If he does eat then go ahead with the insulin injection otherwise it is a no go. A glucose can be fine at 100 and even at 300 depending on the vets comfort level. If it was low normal then yes he could have dropped to an unsafe number. If he is dehydrated which isn't uncommon in diabetic kitties and he won't eat then I do think you should either contact your vet or take him into the emergency center. There are so many issues that can make an older cat ill and if blood work or a urine test was not performed we are pretty much at a brick wall until some further tests are completed. If he is in fact dehydrated then he would benefit from some IV fluids overnight and close monitoring of his insulin levels. In my opinion I wouldn't wait until tomorrow. If he doesn't eat or perk up he needs to be seen tonight.