I have a Pit bull and he eats a good amount of food but he still won't put any weight on in his hind end, I don't know what to do?

Evaluate him using LongLiveYourDog.com - Life Span Study - Rate Your Dog (http://www.longliveyourdog.com/twoplus/RateYourDog.aspx)

Take him and a stool sample to the vet. See if the vet agrees he is too thin. If so, let the vet determine why. If not, there isn't a problem.

You might read through the sticky at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171

Maybe more walking would build muscles back there.

What is a "good amount of food"? How much does he weigh? Take a look at labman's sticky and make sure that you aren't worrying for nothing. Pitbulls shouldn't be fat and should have a well defined bone structure. It is actually better for their hind ends to have less weight as they are prone to hip dysplasia. The bulk of their weight is in the front and the hind quarters should be muscular and highly defined. If you are still not sure, follow labman's advice and take him into his vet's. Let him evaluate your dog and discuss what you are feeding and how much you are feeding. His size and weight should dictate the amount of chow fed.