He eats once a day gets a walk once a day and could his weight be cause by having goat's milk when he was a pup? He must be healthy though because he runs alot, he's very active but he can't jump without his head going back into his necks.
What he ate as a pup probably has little bearing on his present weight. Cut back the amount that he takes in and incease his level of exercise. Also may want to have his thyroid level checked.
What & how much are you feeding him? You could try reducing his food intake and using green beans in place of the amount of kibble that you removed. If you use canned green beans either rince them to remove the salt, or buy the unsalted ones.
How old is the dog and how much are you feeding him?
You should feed according to size and activity. Never feed the recommended amount of the back of the food sack as they are only guidelines. You must adapt for the individual. Walking alone doesn't really do much for a dog. They need vigorous exercise in the form of running and swimming.
How old is he and how much does he weigh? Can you feel his ribs at all?
"Each is a creature of Earth and is entitled to reside on it with dignity"
Have the vet check his thyroid if he's having problems with it that could explain the excess weight. I would also feed twice a day instead of just once, just cut the normal amount in half per feeding.