Since he went below 90, that's a reduction. You have a few options:
1. Drop dose. Push the plunger all the way down as hard as you can, insert into vial, release the pressure. Result is one drop of insulin in the needle, not visible in syringe. When you give the shot, do your best to hold plunger down for 10 seconds while the needle is in, to give time to absorb. Depending on how he does tonight, a drop dose may even be too much.
2. Do a two week OTJ (off the juice trial). Continue to test at the usual preshot times (just no shot obviously), and whenever you get a chance during day/night. The problem with this is his numbers are higher than we'd like to see for remission - cats in a strong, long-lasting remission are pretty much all green all the time (see my SS as example). If his numbers start to creep up that's a pretty good sign he still needs insulin, just Vetsulin too harsh to safely get him where he needs to be. In my opinion, an OTJ trial is worth a shot because it's possible the Vetsulin is sort of working against him in these lower numbers (dropping low, body overreacts and bring him up).
3. Switch to a depot insulin like Lantus or Levemir. ProZinc might work, but I fear it could be a wasted step/money. I would get a single pen of Lantus/semglee (or Levemir), if cost is an issue you can check the supply closet or DCIN.
