If you want to know if your dog is underweight, you can post a few photos (side views as well as top-down) and we might be able to help you figure that out.
How much does your pup weigh, and how much do his sire and dam weigh, if you know?
I'll work off the assumption that your pup weighs between 55-60lbs for now.
In that case according to the manufacturer, he should be eating about 3 cups of food per day (3 cups assumes a 60 lb dog). Usually, food manufactures give you a range (say 2.5 -3.5 cups), but Iams doesn't, for whatever reason.
A range allows for dogs who are more or less active; who have more or less muscle built up (muscle burns more calories); for dogs that are more energy efficient than other dogs (energy efficient dogs just naturally use fewer calories); for different climate and living conditions (cold weather dogs and dogs who live outside burn more calories).
It seems to me that unless Payton falls well outside the weight range I've posted, he's eating a reasonable amount of food. The general rule I follow is if I put down food and my dog is clearly still hungry when he's done eating it, I'll feed him a bit more. (You may be able to go up to 2.75 cups of food, for example or maybe 3). If he walks away not having eaten it all, then I lift the bowl, measure what's left, and then I decrease the amount of food I feed.
I keep an eye on my pups. If they start to look overweight or thin, I adjust again. But my pups always kind of look like bricks for a while, then they shoot up and look thin and gangly, as they grow, so I have to keep that in mind as well.
There is no exact science. That's why I like to bring my dog to my vet (for a second set of eyes and my vet's opinion). It also gives me a chance to weigh my pup frequently, and socialize him to my vet's office as well.
Hope that helps.