Don't change your dog's food without discussing it with your veterinarian first. Diet changes, especially sudden ones, can make your dog sick. His regular food may be fine for him; he just needs less of it. Many people misunderstand or don't pay attention to the feeding guidelines on their dog's food. And even if you have been following the guidelines diligently and measuring his food, it may not be the right amount for your dog. Your vet can advise you how much to feed him.
You may also need to change how often you feed your dog. Most dogs do better with scheduled mealtimes, rather than leaving out food for them all day. If your pup is used to eating often, try feeding him several smaller meals. Avoid giving him table scraps. To keep his diet on the right track, offer food only at mealtimes. If you can't resist giving treats, offer pieces of fruit (such as apples or bananas) or vegetables (like carrots or broccoli). They're low in calories, and dogs love them.
Since overweight dogs are also out of shape, an exercise program must be started slowly. Begin with short walks and work up to longer ones. Once your pooch has lost some weight and built up his stamina, you can intersperse periods of jogging or running. Be cautious about exercising him in hot weather. Go out early in the morning or in the evening, when it's cool, and stop before your dog shows signs of exhaustion, such as panting. Your natural cooling system is better than your his (you can sweat; but he can't), which means you can exert yourself heavily for longer periods without overheating. Dogs will try to keep up with their owners even when they're overheated, so it's up to you to recognize when it's time to stop.
Your dog didn't gain weight overnight, so it'll take some time to get him down to a normal weight. It may not be easy, but getting him healthy can add years to his life.