To build muscle Pitbulls need a high protein diet consisting of 40% protein, 40% carbs, and 20% fat.  You need to source a quality dog food and supplement this with other protein sources such as chicken and fish. You can also add food supplements that are recommended for Pitbulls. Building Muscle in pitbulls consists of the right exercises, diet, and supplements. Feeding your dog 6 small meals a day will put them in an anabolic muscle building state and ensure they are getting the best possible diet for building muscle.

Links:

5 Tips on How to Care for a Pitbull

How to Make Your Pitbull Muscular in 3 Steps