First the dog should be gotten into his proper weight. This will vary from dog to dog, depending on his height and density of bone. Measure his food out every day. Obviously, if he needs to diet, he should be fed a bit less until the correct weight is achieved. Any serious exercise program should not begin until a dog is over a year of age as everything is still growing before this and damage could result if a young dog is pushed too hard and too quickly.
All dogs are different, and some require more effort to get proper muscle tone than others.
One could start out with nice little walks, keeping the dog at a brisk trot, gradually increasing the distance until the two are going anywhere from 2-5 miles a day. Going uphill is especially beneficial, as it develops the rear more, and keeping the dog on a softer surface such as dirt or grass is much easier on his structure than pounding on concrete. Teach the dog to retrieve and throw a ball for him to bring back. The dog is called first up, and then down, the stairs with treats given at either end to make him race from one handler to the other. When he's gotten the game figured out, eliminate the treat at the bottom of the stairs and only give at the top.