This is pretty straightforward. Eat less, exercise more. Don’t feed any treats that are store bought, feed veggies or fruit. Change your dog’s food if it is a low quality food. Feed at a schedule twice a day with the proper amount of food for your dogs weight and activity level. Buy a measuring scoop if needed. It is very important you are not overfeeding him. Separate your dog from other dogs during feeding time so there is no possibility of sneaking food. Starting walking/swimming/playing ball. A good 30 minute slower paced walk (to start with) will not hurt him.