Dieting doesn't work on its own without hard physical exercise, neither for people, nor for dogs or horses. Hard intellectual and bodily load on a daily basis - is your answer for both dogs. Too little of exercise leads to depression, and
any issues with stress or anxiety? My vet said
is true, one thing helps the other: overweight or weight loss helps the stress, and stress helps your dog to lose weight or to put more.
Besides increasing number of times you throw his ball for him, feeding him strictly at the certain hour, try to start with "You wouldn't have it, if you don't pursue the task". Hide his meal and ask him to find it, nothing should be for free. Thus you would revive his primeval urge, food doesn't come to wolves so easily, they have to track it first.