There are cases where adopted dogs shows fearful aggression just because of their previous bitter experience with past owners or on any incident that hurts them. It will take sometime for them to adjust new environment and showing stick or hitting them may lead things go even worst. First thing is gain trust from your puppy by playing with him giving treats often (marie biscuit is enough) and providing his favorite toys, giving raw hide bones. If still he starts biting instead of scolding him or showing stick just ignore him i.e. you can go to a room and lock that room, so that your puppy feel alone for sometime and that makes him to understand the things which is not acceptable.