Nipping @ Other
Socialize him with dogs that you know are friendly and not likely to retaliate with aggression. He is showing fear/anxiety meeting other dogs because he hasn't been exposed enough at an earlier age. If you could enroll him in some obedience classes and arrange play dates with some of the mellower dogs or ones in your neighborhood that wouldn't frighten him that would allow him to gain confidence. Grasping his mouth closed when he nips will not help, you want to ignore negative behaviour and reward positive. If you could meet others in a fenced area then he could observe them at his own pace and begin to feel less threatened. At his age he won't take long before he's engaging in play.