Hello, I'm sorry that you're having trouble getting your kitty to eat. Anorexia is very serious and needs to be treated proactively. Here are some tips you can try at home to get him to eat:
http://cats.about.com/cs/healthissues/a/fatty_liver_2.htm Another option is to syringe feed your cat. There are a ton of videos on youtube that demonstrate how to do this. A high calorie food like canned Wellness Kitten, Chicken, or Turkey flavors is good because you can get more calories in your cat for each feeding.
How long has he not been eating right? Not eating can also be a sign of other conditions and you should take him to the vet if this has been going on for a while. Sometimes cats can stop eating if they are stressed, but it can just as well be a sign of an illness. I'm a bit concerned because your vet has not put you on the best treatment plan for your cat's diabetes. NPH is not a good insulin for cats, and not many do very well on it. The current recommended insulins are Lantus, Levemir, or Prozinc. Lantus works very well in cats and has a very high remission rate. Also, diabetic cats should eat a low-carb, canned diet only. The canned Friskies you have is ok to feed as long as it's low carb--the Friskies pates are all good, but you don't want to feed anything with gravy. The dry food is very bad for diabetics and will keep blood sugar levels high--kind of like if a diabetic person ate a lot of donuts and cookies. Most cats' blood sugar levels drop 100-200 points when the dry food is removed.
One thing that I am concerned about and I think you should rule out right away is Diabetic Ketoacidosis. This is a very serious, life threatening condition that can happen if a cat has had extended high blood sugar, an infection and dehydration. You can run out to any pharmacy or Walmart and get a container of Ketostix. These are urine test strips that can show you if there are ketones present in your cat's urine. A cat in Diabetic Ketoacidosis needs to be rushed to the vet immediately.
Another thing that would be good to pick up is a home glucose meter and some lancets (26-28g). Any human meter will do--Walmart makes a good, very inexpensive one that a lot of people use here, and the test strips aren't as expensive as many other meters. Home testing your cat's blood sugar is key to helping him get better. Testing at the vet is pointless and a waste of money--the results will be inaccurate because most cats get stressed out at the vet and their blood sugar goes up, and dosing needs to be adjusted based off of daily testing, just like people do.
I know I just threw a LOT of information at you, so please post back and ask more questions. There are a ton of knowledgeable people here ready to help you and your kitty.