When he nips, shout "ow! no!" a bit louder and sooner than necessary, and if necessary stop playing and walk away. That's how dogs teach each other not to nip, after all -- they growl or bark and don't play with the annoying one until he learns not to be so rough.

"Pit bulls" should be as capable of being gentle/careful as any other dog. But a puppy is a baby; as surrogate parent, it's your job to teach him how to be a Good Dog.

Oh -- and give the pup some good chew-toys.