[Help] Why is my 4 month old puppy so scared of other people? (including my brother and neighbors)

We got him when he was around 3 months old and he has already trusted me and my mom 100%, but not anyone else. He doesn't bark at the people, he just runs away and sometimes pees from the fear. What do you think I should do? via /r/dogs http://ift.tt/2rhBcHe