Pit Bull is Nippy with Guests?

My mom has a Pit Bull and two other dogs. We got him five years ago from a shelter and he was underweight and not healthy. He now is happy and healthy and loves my mom. My brother and I have moved out for college and he now lives with my mom. He follows her around the house and he is getting very protective (possessive) of her. Now whenever she has any friends over he stares at them and watches their every move and if they move a chair or get up too fast he will nip them.

No one wants to come over anymore and we don't know how to make him stop. He is never aggressive towards me or my brother, but he intimates everyone else who comes over and he takes full advantage of it. What should we do?

By Lily D