
I have never found the diet foods a lot of usead at the recomended amounts would actually put weight on my dogs.
I found the best method is to feed a good quality food, but give half rations bulked out with vegetables, Carrots, Cabbage, unsweetened Pumpkin (very popular for some reason in USA) Green Beans etc. Basically the vegetables won't be diogested, buty will make the dog full so that he is less inclined to scavenge.
I have a breed that easily puts on weight.
Took me more than 3 months to get weight off one of mine after she had been over indulged by my Dad on a fortnights holiday.
Once ypou have the weight down increase the food to 3/4 of what you think he should have and monitor for a two week period. If he continues to loose weight then up it a timy amount until his weight settles.
Then I would weight the food and find a scoop (empty Mullerice pot, Margarine tub etc) that takes exactly the right amount. If you change foods weigh again as different foods weigh differently. By having a measure like this you can shake a bit of if you think he looks too prosperous or not being so active, and round it up if he needs a bit more (like while moulting or under more stress and strain).
I don't weight my dogs much now as have amanged to know just by regularly feeling them all over if they are in the right condition and weight, but before I got practiced at this I would take them into the Vets monthly to monitor their weight, so that I could stop the rot before it got too bad.