My first thought is to take a look at the food you are feeding him. Many commercial dog foods have high calorie filler. If you switch to a lower fat food, it will definately help and some of the human grade foods have more nutrients so they have more energy. You can also add things to their food that will satisfy their hunger without adding calories, such as boiled or raw chicken (make sure you cut all fat off) or canned green beans for example.

Of course there is no substitute for good old fashioned exercize. Take him on long walks - if you see another dog, cross the street. Get a Halti or a Gentle Leader collar if you have trouble controlling him. Take him to a park during the off times and go for a run or a brisk walk. Take a ball and let him run and fetch. Make your own agility course in the back yard with cinder blocks and old broom sticks and train him to jump the hurdles.

Just like people the key to losing weight is low fat food and less of it and increased exercise. Then be patient - it took a while to put the weight on - it will take a while to take the weight off.