First off let me say I am deeply sorry that you have decided not to treat you cat for this very managable disease. 9 years old isn't terrible old as my own sugarcat is approx. 12 years old and doing wonderfully and you wouldn't know him from my 3 and 4 year olds.
Should you rethink you plan at the moment and decide to give insulin a try, let me tell you if you do as we do and test him at home you will not be dragging him back and forth to the vet's all the time. You can do everything they would do for him from the comfort of his own home, and simply keep a spreadsheet of your testing to confer with your vet either over the phone or by email. Home testing is very simple, we obtain a small sample of blood from the edge of their ear and test it with a regular human glucose meter. We test before every shot to make sure they are high enough to give insulin, and run our own curves periodically to check on how the insulin is working, then I personally just email those curves to my vet.
As far as food goes, many of us feed 9-lives, Friskies, or Fancy Feast all which can be found at Wal-mart, If you go here
http://binkyspage.tripod.com/canfood.html and look at Binky's list pick those that are 10% carbs or less, personally I feed 10 cats and only 1 diabetic but they all eat exactly the same thing, Friskies Pate Flavors. Currently on this diet my sugarcat is diet controlled, but it did take him a couple of weeks of insulin besides the diet change to go into remission.
However I'm going to be very honest here, even if you switch diet on your guy, if it isn't enough to get him into remission you are setting him up to literally starve to death with full bowls of food in front of him as well as several other nasty side effects including, blindness, DKA a deadly conditon of keytones in the urine from untreated diabetes, and nerve damage to his legs.
IMHO If you aren't going to treat this disease which is very easy to treat then do the kindest thing for your guy, and either have him put down or look for another home for him where he can be treated appropriately. However, also know that placing an unregulated cat in another is very very difficult as most shelters or rescues will not take them, or will immediately put them to sleep.
I personally adopted my current SugarCat Max from this very board as a diabetic, after losing my first FD cat, if this wasn't such a very easy disease to treat or effect a cat's quality of life in the least I would not have set myself up from more heartbreak by willingly adopting a diabetic cat. But had I not, I would not right now have a wonderful, loving and happy boy purring in my lap as I type.
Mel, Max & The Fur Gang