I feed my boys (80 lbs each is about perfect) three cups per day of Fromm (Pork and Pea) for maintenance and they get some training treats (light string cheese, cheerios, dog treats, cooked meat), sometimes a TB of wet food as a topper, as well as veggie and fruit scraps. They are five and two and both gain weight easily, especially the five year old. They wrestle on and off, Linus gets trained and I walk them 30-60 mins per day, most days, plus they get out for a good run a couple times per week, depending on the season.
Being that your dog is much younger and still growing and filling out and he looks like a field Lab so is probably leaner and more energetic plus he gets more exercise than my dogs, I'd say you're not feeding him enough.
The simple answer is, if he's not heavy and you think he is hungry feed him more. It won't hurt to add more food to his daily ration and see what happens. Just go up a quarter cup every few days and see how he acts.
You could also ask his breeder, she should know the needs of the dogs she produces.