Lots of dogs, some related by breed, others random dogs can have protective traits.

Even little chi's can try and gaurd their owners from other people/things. Some breeds are more well known for it then others.

How old is your brother, if he's young and inexperienced with dogs, some big ol' gaurd dog would not be his best option.

Any dogs you see that you like, make sure you do weeks, if not months/years worth of research on the breed, and dog training/dog behavior. So this way you will not be ill equipped to deal with a dog from the get-go.

Here are some sites with Dog Breed Selectors, that are a simple GUIDELINE to help your brother pick, what dogs might be most appropriate for him.

http://animal.discovery.com/guides/d...r/selector.jsp

http://www.dogbreedinfo.com/search.htm

http://selectsmart.com/beta_dog/