Is you dog eating a species-appropriate diet? By that I mean, a diet high in quality meat protein without heavy amounts of grains or fillers?
Most dogs who are put on grain-free diets gain lots of muscle and lose their excess fat. It is what their bodies thrive on, and it's healthy for a LOT more than just their body wieght!
I recommend going to your local feed store/pet specialty store (most pet supermarkets like Petsmart don't carry grain free) and looking for Orijen, Acana Provincials, Taste of the Wild, Nature's Variety Instinct, Wellness CORE, etc. These foods have excellent ingredients and will therefore cost more than grocery store brands, but you will be feeding half as much food and you will save in Vet visits.
It's a good idea to add fresh, wholesome foods to your dog's diet, as kibble is so calorie-dense. Green beans can be added to fill your dog up, although they aren't really that nutricious. Add fresh meats (raw or cooked) in place of part of your dog's meal, canned salmon, chicken, and other lean meats are best. Add cottage cheese, goat's milk, greek/plain yogurt, egg and other healthy dairy. Puree a blend of healthy veggies (leafy greens, broccoli, tomato, etc) and add that in. Just like with people, dogs thrive on as much fresh, healthy food as possible, in addition to a balanced, species-appropriate kibble.
Exercise is key! Healthy, natural exercise. Running and exploring in safe off-leash areas. Playing with other dogs. Frequent, short, brisk walks. Keep his metabolism active through out the day.