Many dogs "act hungry" because they're genetically programmed to eat as much food is available -- some will self regulate, but they're less common than dogs who will scarf as much as they can get. So don't listen to him.
It's incredibly important that you learn to evaluate body condition -- your vet can teach you, but there are charts easily available online. THAT is the right way to know if your dog is too lean or too fat (or just right). This may get you started: