If he wants a dog that is protective, that could be any dog of any breed or mix. If he wants a guard dog to toss outside 24/7 then a Pit Bull is a very poor choice. (Actually, any dog is a poor choice because a guard dog can't stop a bullet, or baseball bat, or resist poisoned meat thrown over the fence.) He would need a guard dog to keep his Pit from being stolen. How old is your brother and why does he want or need a guard dog or Pit Bull? Pit Bulls and guardian breeds need mature owners to responsibly care for them and keep them out of trouble, not just to impress their friends with. Given the description of what he's looking for a Pit Bull sounds like a good match, but they can require a lot of exercise depending on the individual dog's drive, because these are terriers. Maybe a more bully breed like American Bulldog, Dogue De Bourdeaux, English Bulldog. Like others have mentioned he needs to do lots of research to find the right breed.