It should say on the bag how much food per weight and age you should give him. Also, I would switch to a different food. Purina isn't a very good brand and you can get better food for the same price or less. It doesn't say what type of animal is in the fat, digest, and byproducts, it is mostly grains, and contains menadione, a chemical linked to liver toxicity. It just has all sorts of stuff that have no place in a dog's diet.
Start with what the bag suggests and then adjust accordingly. If he gets skinny give him a little more and if he gets pudgy cut the food back a little bit. And yes, Purina 1 is not the greatest of foods.
How much does he weigh? I have a 16lb dachshund mix (who may have min pin) I started off with 1/2 cup 2x a day, then went to 1/3 and now we're on 1/4. So it takes some tweaking. My poor guy LOVES to eat, he thinks I'm trying to starve him