
Just like with people weight gain can only be controlled by the animal/person consuming less fuel than it needs to sustain itself.
a lot of people and dogs start gaining wieght once they become more sedentary and less active, and it becomes a vicious circle as the more they weigh the less inclined and able they become to excersise.
From someone who has been overweight a lot of her life, and is at present adressing the issue I feel I can speak from expereince.
The dogs food needs to be cut in half, and to stop him feeling deprived and hungry (and then leading him to scavenging) the other half of his food can be made up with vegetables, like carrots, green beans (in US unsweetened tinned pumlin is a favourite). All treats should be stopped or replaced with pieves of raw carrot/apple.
On of mine stayed with my Dad for two weeks and with all the leftovers he gave her she gained weight which took me sevearl months to shift with half rations, so for meaningful weigh loss it really does mean halving the correct amount of food (I suspect this dog is getting more than the correct amount now).
Most advice on amounts to feed exagerates the amount needed by around 1/3 in the expereince of most dog folk I know, and with my own breed I only feed around half of what they advise anyway.