I've noticed that since I began feeding my overweight boy a combination of dry and canned food that he gradually slimmed down - I believe the canned food has really helped him to feel more satisfied on less food. If your cat is on a prescription diet, your vet may carry a canned verision of your cat's low magnesium diet. I've seen some articles that say canned food is better for a cat with urinary problems anyway, so introducing canned food may help on both fronts.
If he's on a prescription diet be sure to check with your vet before making any diet changes.
If you're interested in a raw diet as mentioned above, it would be a good idea to consult a holistic veterinarian about your cat's special needs first.