Maybe I can help with the oz.. this is a lot of information to take in at once so you must prioritize. IMO the order of priority should be:
1. He must eat. Priority number one.
2. Try to get him to accept no more dry. switch him to wet cold turkey. he may balk, and you may have to trick him by crushing some dry over the wet, but he's off insulin so now is the time. ***important*** if he refuses to eat ANY wet food, use Dr. Lisa's transitioning tips (on same site i gave you).. it IS important that he eat so if he goes even a short length of time refusing food, you'll have to go to plans b, c and d -- see priority 1. keep us apprised, we'll help.
3. Mix as much water in the food as you can get away with, it will help with the full feeling and it will also help keep his system healthy coming off insulin.
4. Diabetes. This is extremely important, and while it sounds absolutely wonderful that he may be off the juice, you must NEVER let your guard down. He must be tested regularly, which thankfully you can do at home. Every day at first, every week later. You see dandruff, you test. You see him drinking water, you test. You see him laying around looking too tired, you test. You see ANYTHING out of the ordinary, you test. It might not be gone, and if it is gone, there's a good likelihood it will be back.
Now......... Maximum oz of food per day.. he is 26lbs. Current thinking is he needs 15-20 cals per lb per day at 26lbs = 520 +/- calories to maintain weight, which we do not want. (
http://binkyspage.tripod.com/foodfaq.html) Soooo.. taking 1 can of Friskies (one of the least expensive commercial foods) Special Diet Ocean Whitefish Dinner and we find it has 184 calories in the 5.5oz can (
http://binkyspage.tripod.com/CanFoodNew.html) That means a little under 3 cans per day will maintain that weight. You could start at say 2.5 cans per day, heavily diluted with water. Note: We can talk about cat food addictions later, when it comes to fish, ultimately you don't want to feed a lot of fish, but when moving a cat onto canned, sometimes it's helpful. Also do NOT buy any "in gravy" or "in sauce" foods or you'll never get him to eat the healthier pate varieties.

Remember: the mere switch to canned is gonna get that weight loss started for you.
Hope this gets you off to a good start!