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.