I'm so glad you are switching his treatment to insulin. Basically, Glipizide work by forcing the pancreas to produce more insulin. This method of treatment has a few drawbacks:
1. it can stress, or even burn out, an already stressed pancreas, reducing the chances of remission and leading to permanent insulin dependence
2. it is effective in only about 1/3 of diabetic kitties, and can waste valuable time in getting the blood sugars under control
3. there is a significant incidence of liver damage in cats who use oral hypoglycemics (so you may want to have this checked at Frankie's next vet visit)
Since you will be BG testing, I don't see a problem with just diving in and starting insulin right away. And huge thank you for taking on the care of this sweet boy.