this website
http://www.nutroproducts.com/ncdoglbpuppy.asp has a chart that shows approximate feeding amounts for nutro natural choice large breed puppy. The amounts are based on the puppies age and weight. There should be a similar chart on the back of the food bag.
The ingredients for this food are:
Chicken Meal, Ground Rice, Corn Gluten Meal, Wheat Flour, Rice Flour, Rice Bran, Dried Beet Pulp, Poultry Fat (preserved with mixed Tocopherols, a source of natural Vitamin E), Natural Flavor, Sunflower Oil (preserved with mixed Tocopherols, a source of natural Vitamin E), Lamb Meal, Potassium Chloride, Choline Chloride, Dried Egg Product, Lecithin, Dried Kelp, Vitamin E Supplement, Ferrous Sulfate, Zinc Sulfate, Zinc Proteinate, Biotin, Ascorbic Acid (source of Vitamin C), Manganese Proteinate, Glucosamine Hydrochloride, Manganous Oxide, Chondroitin Sulfate, Niacin Supplement, Calcium Pantothenate, Copper Sulfate, Vitamin B12 Supplement, Vitamin A Supplement, Copper Proteinate, L-Carnitine, Sodium Selenite, Riboflavin Supplement (source of Vitamin B2), Pyridoxine Hydrochloride, Thiamine Mononitrate (source of Vitamin B1), Vitamin D3 Supplement, Menadione Sodium Bisulfite Complex (source of Vitamin K activity), Calcium Iodate, Folic Acid.
Ground Rice, Corn Gluten Meal, Wheat Flour, Rice Flour, Rice Bran(Five of the first six ingredients) are all going to be hard for your puppy to digest, as well as potential allergens. These are all cheap filler ingredients that provide basically no nutritional value. I would guess they are all going in one end and straight out the other, which is why he is pooping so much.
If you feed a higher quality food, that does not contain these types of ingredients, you would probably see a drastic reduction in his poo production.