If you look at the ingredients in Science diet you will notice it is is mostly grains, grains are a carbohydrate, the body converts carbs to sugars which goes on as fats,
where as proteins are made up of amino acids, which the body uses for cellular repair and to develop muscle
fats, help to hydrate the body, and animals need fat particularily animal based fats more so than humans
EVO is a low carb diet, which is the reason it is effective for weight loss, it will help to increase muscle tone and your cat after a bit will likely become more active as a result which will also help to tone him up as the fat is lost.
Cats are true carnivores so all that grain is not good for cats, and because carbs convert to sugars can also end up causing diabetes, they need meat based diets not grain based proteins. Most of the nutritional studies that vets are taught in school are provided by Hills, which is really vastly out of date and unless the vet are upgrading their knowledge on nutrition and looking at sources other than Hills for info, they remain in the dark ages.
Science Diet is still following protein research studies from 80 years ago research done on rats(non carnivores) instead of getting getting their head out of the sand and looking at the newer research that has be actually done on the protein needs of cats and dogs
but you may have to reduce the feeding amount to less than the 2/3 cups you currently feed as it is nutrient dense. I have a 84 lb greyhound eating the EVO canine version who was also suffering from weight gain on a very low protein, low fat diet, I switched him then to EVO, where he lost weight and is now maintaining weight he eats 1 1/2 cups per day less than recommended for his size
If you decide on EVO for feeding amount , my suggestion is to determine how much he should weigh and go by the inactive feeding amount using the feeding wizard, and tailor from there to his needs.
http://www.naturapet.com/display.php...wiz&animal=Cat