I have a 5 month old Corgi puppy, that has not been neutered yet, who has been very nippy since we got him. When he was younger he would mouth and nip but would break the skin occasionally due to his teeth being so sharp. When we play with him on the ground he will mouth our arms and hands which can escalate into a more aggressive nipping. This will occasionally result in the puppy going behind us and "attacking" our backs. During these fits he does not become deterred with toys or treats and growls while lacking on to body parts. When this happens we have tried walking away, making a noise to connote rough play, and even time outs to have him calm down. This aggressive behavior does not seem to be getting better and we are getting concerned. He does not seem to like being pet and will mouth/nip at people who pet him when he is not in a mood for it. He is a well mannered dog who is in socialization classes and obedience classes but is this behavior normal herder puppy play or is this an aggressive dog who needs an intervention?