Roco, please dont take offence but at 40kgs a lab is overweight. Start him overweight and he could end up like Harry who comes to the Canine Fitness Centre and weighs 80plus.
Do not follow the feeding instructions on any dog food bag as every dogs metabolism is different. In your case I would cut his kibble in half and bulk it out with cooked pumpkin until he looses at least 5 kilos. You can then cut down on the pumpkin andincrease the kibble a cup at a time until you can maintain his weight around 30 to 35 kilos.
Because Labs are cold climate dogs, they have a genetic propensity to turn excess food into fat so that they do not get cold. But with our temperate climate they do not need that extra layer of fat so make sure you are not overfeeding him.
Here at the kennel we use Pro Pac and Earthborn. Our own dogs are on Earthborn Coastal Catch or Primitive Natural. My biggest dog, a BC at 25kilograms, gets 1.5 cups of kibble a day, with a turkey neck from time to time. If he goes through a highly active phase and starts dropping weight , his ration is increased to 2 cups until he is back up to his best.
Border River Pet Resort