Black Lab loose stool

Question
I have a large black Lab, at one time weighed 141lbs, after a 6 month diet he weighs a svelte 98.7 lbs. Currently I am feeding him about  6 cups daily of Diamond Lamb and Rice formula. Unfortunately he poops a lot and most often it is very loose. How do I get his stool firmer and perhaps less often?

Answer
I would drop him to 4 cups, maybe even 3.  The reason he is pooping alot and it's loose is because he's eating too much food, can't utilize it properly and just expels it.  

I have a 100# boy who eats 1 cup for each of his 2 meals, with a little canned, and he has normal firm stools, and goes once a day.