Cats can get fat enough on their own. If he is not losing, reduce the ration again. Continuing this process will usually result in weight loss. It will also result in a hungry dog, and therein lies the rub. Some people can't abide a hungry dog.
Some hungry dogs are downright unabidable.
There are dog foods, some of them available by prescription only, that may help. When you try that technique with a cat, he often simply becomes less active. He sleeps twenty-two hours a day instead of twenty. I think with some obese cats, if you starved them to death, they would die fat. And food deprivation can lead to a serious liver disease in an obese cat.