Your vet will have an illustration to help you determine where Buster is as far as the need for taking off some pounds. They can also give you his caloric needs, which is a good way to decide the amount of any food you give him. The weight control foods, especially grocery store brands are notorious for packing on the pounds.
You might want to make sure you are feeding him the best food you can afford, check with this site for premium foods, these are usually found in smaller independent stores rather than the larger stores. However, Petco now carries Wellness which is a quality food.
Check out this site:
http://www.dogfoodanalysis.com/
My big boy Turbeaux was always a thick log on legs, and I made the mistake of feeding him Iams weight control, he just continued to gain weight. When I switched him to a premium food, he is now 26 pounds of muscle, well defined waist, glossy coat and a lot of energy. He is almost 8 years old and looks & acts wonderfully healthy