Fancy feast canned food is just fine to feed a diabetic! I would recommend limiting the fish flavors to once a week, but there are lots of other flavors out there you can feed. Any Fancy Feast that says "classic" on the label is diabetic safe. Avoid the other Fancy Feast flavors (the gravy makes them high in carbohydrates, which is bad for diabetics).
There is nothing about prescription food that makes it better than commerical food, and most of them aren't even diabetic safe even though they are labeled as a diabetic diet. The only prescription food that is appropriate for a diabetic cat is Purina DM canned; all the others are too high in carbohydrates. But even the DM is overpriced and unneccesary, because it's pretty much the same thing as Fancy Feast pates (the "classic" line). You can return your unopened cans back to the vet for a refund. Just tell your vet your cat refused to eat it.
You can also check out the list that Larry linked above for other options besides Fancy Feast. How old is Louie? I would suggest another food instead of Fancy Feast if Louie is an older cat, or has a history of kidney disease.
What insulin is Louie using? Has your vet taught you how to home test him? If not, there are some great resources here that can teach you. Home testing is not only safer for your cat because it allows more accurate dosing adjustments, it also is way less expensive than bringing him to the vet to be tested. So double bonus.
Hang in there! A diabetes diagnosis seems like such an overwhelming thing, but once you get the hang of things, it becomes much, much easier. The three key things that will make your life much easier is having Louie on the right food, the right insulin, and home testing.

And diabetes is not a death sentence, and on the right treatment path cats live perfectly normal, healthy lives. Bandit has been diabetic since the age of 7, and he'll be 14 this January. And he's still doing great!