I believe that question is answered on the side or back of the kibble bag, if you feed canned, check out a kibble box or bag at the grocery store. I just read my bag and it says for a dog your size and weight, to feed him 50 k or 4 to 5 cups of dog kibble and whatever you mix with it. Happy feeding.
I feel that the best place to get this information is your dog's veterinarian. Also, Labman has a good point too. His answer sounds like the information on the side of a bag of dog food or someplace else that I saw feeding and dog fitness and shape information. I think Labman and the vet are your best bets.
First of all, if you have a TRUE American Pit Bull Terrier, at 8 months of age, he should NOT be weighing in at 85lbs. More than likely you have an off-shoot of a APBT called an American Bully. Full-grown male APBTs at the age of 2-3 years old shouldn't weigh more than 70lbs. There is your first problem.
Second, do not go by what the back of the bag says. That is an assumption by the pet food maker and also a marketing ploy by the manufacturer to get you to feed your dog more, so that you buy more food. If you feel that you have to go by the bag, feed a bit less than what the bag states. Better yet...CONTACT YOUR VET and ask them. They are the professionals, after all. They can provide you with a baseline on what you should feed.
APBTs and their relatives are known to have skin and allergy issues, so going with a high-quality food (preferably grain-free) may help. Avoid the "common" meats such as beef, chicken, and lamb as many dogs through breeding have acquired allergies to common meats. Go with more "exotic" meats such as venison and bison. Fish is also excellent for dogs, as it contains omega-3 and omega-6 to help with skin issues. Also, many owners "switch up" the food every 3 months to help avoid allergies.
Best course of action for you...contact your vet. If you don't have a vet...GET ONE NOW!
For what it is worth, I have a 53lb female APBT/whippet/lab mix, and she gets 1 1/4 cup of food twice a day. She get a bit more in the winter. She is on a fish/sweet potato diet and is allergy-free (which she wasn't when we adopted her) as she was being fed Science Diet. ICK!
I have a 62 lb Stafford shire who is very active. We feed her 3 cups 2 times a day. I also have a 45lb American pit who also eats the same. I do encourage though that you are careful on what you feed your dog. Our vet told us to look in the ingredients and choose something that does not include bi products. Also a food that contains lamb is very good for them.
