As obligate carnivores, cats are very able to break down both fat and protein and extract the molecules necessary to make glucose. Humans can do that some, but not as well, since we are omnivores.
if his pancreas has healed, he ought to be able to handle some carbs. i don't know the exact answer to your question, but we have often found that some cats do better getting regulated with at least a few carbs.
Dr Lisa, a vet, has a great site on feeding cats and cat health called http://www.catinfo.org. She posts on this site sometimes. A cat's natural diet is a bird or a mouse, and both of those would have a few grains in their stomachs, meaning that they would contain a few carbs.
I'd guess that most of us feed around 4-5% most of the time. In a cat with a functioning pancreas, zero carbs shouldn't be necessary. you probably wouldn't want to give Freddie more than low carb (which means less than 10% or so).