I promise not to yell. 
Obesity in cats is really serious, far more serious than in dogs or even in people. Obese cats frequently become diabetic (of course this happens in people too, but it's more common in obese cats).
You will never take the weight off this cat if you don't feed him separately from your other cat. You can't free feed an obese cat. You can't give him a chance to get at the other cat's food. You have to show that yes, you really are smarter than the cats are and create a schedule of feeding and separation. I don't know why so many people find this so overwhelming an idea, but many do. I promise you there is no alternative. He has to have his food and only his food, period. I swear it. <G>
OK, that raises the question of, which food?
I am not a believer in feeding "lite" foods or diet foods to cats. What you will find as you look at the most recent veterinary research is that "Catkins" diets are proving to be the most effective in managing feline obesity. Whatever your thoughts on the Atkins diet for PEOPLE, for cats, there really is no question that it's the way to go.
Cats have no dietary requirement for carbohydrate AT ALL. None. So any carbs ... grains, veggies, whatever .... that are in their food are just there to provide calories. I'm sure you can figure out that an obese cat doesn't need anything in their food that provides nothing but calories. <G> Stick with the ingredients that are essential to a cat's life, not the ones that aren't. That would be fat, protein, vitamins, amino acids, minerals, etc.
There are some commercial diets that are VERY low in carbs. I am not a fan of kibble for cats, and all kibble has a certain amount of starch in it, in order to hold its kibbled shape. I have heard that Innova's "Evo" cat food contains no grains and is the lowest in carbohydrate of any dry food. If that is true, it might be enough to fix this kitty's metabolic problem as long as the amount the cat is being fed is controlled (not too much and also not too little, as that can cause its own problems, including weight GAIN).
Another good food is "Serengeti" by Timberwolf Organics. It is also lower in carbs, but it does contain grains. Still, again, it might be enough for this cat. I noticed there is actually a link about this food here on this website: market.cathobbyist.com/detail.php?cat=10&de=352
However, my preference would be either a canned food or a homemade diet.
Nature's Variety has a canned food that is 95 percent meat and is complete and balanced. It is my first choice of all commercial canned cat foods. They also make a raw frozen food that contains no grains, that I would highly recommend also. There is a link to this food here too: market.cathobbyist.com/detail.php?cat=10&de=361
However, my REAL preference is a homemade diet. That is how I have fed my cats for nearly 20 years now, and it's the best way to go. You have the ultimate in quality control.
There are several completely grain-free recipes for cats in the book "Home Prepared Cat and Dog Diets: The Healthful Alternative" by Donald Strombeck DVM PhD. I also like the recipes in "Dr. Pitcairn's Complete Guide to Natural Health for Dogs and Cats," although some are way too high in grains. One he has called "Fatty Feline Fare" is quite good and very low in carbs. Both those books are in the holistic cat care bookstore on this site:
www.cathobbyist.com/books/?sub_sect=5
Good luck!
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Christie Keith
Caber Feidh Scottish Deerhounds
Holistic Husbandry since 1986
Meet the Felines!
