Contrary to popular belief, cats should not be allowed to "free feed", as someone else mentioned here. Feeding regular meals at set times each day is much better for your cat to prevent him from over-eating. It also aids in digestion as the stomach only has a small amount of food to deal with at a time, often preventing vomitting. My cat Flip was also 17 pounds last summer. The vet handed me a tiny scoop, 1/4 cup in size, and told me to feed him twice a day, one scoop each, so 1/2 cup total per day. As it worked out, I would feed him 1/4 in the morning, 1/8 in the afternoon and 1/8 in the evening because he would bug me before I went to bed but it still worked out to the right amount. In 6 months, he lost 3 pounds.
There's no doubt in my mind that your cat will lose weight if you cut him back like that but the best way to get your cat to lose weight is to ditch the dry food completely. I would have done it sooner if I knew how bad dry food is for cats. It's full of carbohydrates and other ingredients that do more harm than good for your cat. Canned food contains much more protein and less carbs and if you can afford a good quality canned food, you can find some that contain no by-products, corn, wheat, soy or other fillers that the pet food companies use in their less expensive products. You'll spend more money but in the long run, there's evidence to show that you will save that money in vet bills down the road. Besides leading to obesity in many cats, dry food can also cause urinary problems. When your cat eats dry food exclusively, he is likely not taking in enough water since cats do not naturally seek out sources of water (in the wild, most of their water would come from eating their prey) and is in a constant state of dehydration to some degree. Canned foods provide cats with much more moisture along with the added nutritional benefits.
Flip now eats canned food exclusively and along with losing an additional 2 pounds, he is much more active, he produces much less waste than he did before because his body actually uses most of the food since it's nutritional, and his coat is extremely soft. However, if you must feed dry food, limit him to 1/2 cup per day and you should see his weight improve. If you're interested in learning more about the benefits of feeding your cat a canned diet, visit Lisa A. Pierson's website for more information: http://www.catinfo.org/