For a dog that has an easy time gaining weight the recommendation is light diet feed or something with a larger amount of carbohydrates. Since a higher protein diet needs to be followed by fat, the dog gets more energy and nourishment through a small amount of food. Food with carbohydrates have less fat and therefore the dog can eat a larger amount without a larger nourishment intake.
Also mixing your current dogfood with empty calories, such as shredded carrot and some canned dogfood, can help. Your dog will then be able to eat the amount of nourishment required to keep a healthy weight without having to starve because of a reduced food intake.
This should be accompanied by exercise to reduce current fat levels. High pace long promenades are really good for losing weight. Running around in the woods, having to lift his/her feet over rocks and logs, climbing etc. are really good activities as well. As long as you gradually introduce him/her to it so that the dog doesn't over-strain and other health problems occurs.
Furthermore, you can see if your dog is overweight by touching his sides and see if you can feel the ribs. They're not supposed to be prominent but you're not supposed to have to dig around for them, instead just run your hand along his side and feel them lightly.
You dog is also supposed to have a hour-glass shape on his waste, behind his ribcage the body should become thinner down to the waste and then become thick again, following the rump. If your dog is like a sausage and you can't feel the hour-glass shape easily then he is, indeed, overweight.