How much should I feed my puppy?

My dog is 8 months old and our vet told me to feed him one cup of food a day. A friend of ours recently commented that I do not feed my dog enough. Is this true? How much should I be feeding him?

When you stand over your dog you should see an indentation (a waist) at the end of his rib cage. With him standing, you should be able to feel his ribs when you run your hands over them, but they should not be readily visible when you are looking at them.
   
If he passes these two tests chances are he is at his ideal weight. If not you might want to increase his food until he reaches a good weight.
   
You didn't mention what breed he is or what his current weight is.
   
There are a couple breed exceptions to the above rules of thumb, so if you need more info you need to let us know a bit more about your boy.

He is a bichon yorkie mix. He weighs 9 pounds right now and is not suppose to exceed 13 pounds. He is shaved completely right now so if I rub my hands over him I can feel his ribs, however they are not visible to the eye.