the first thing to you need to understand is this is how dog ( puppies) play you just need to correct the behavior consitantly. My pitty was very simmular when we first got him @ 4-6 months we satrted by correcting this mouth play with a grasp to the top of the neck/head just firm enought to provided pressure not hard enought to hurt this simulates what another dog would do to let him know that thats enought. it may take time but it is worth it in the long run.
