So I am imagining a scenario where you, new people, and he are in a conversation and he starts stuttering. When he is in the middle of stuttering or blocking, nodding to signal that you're still listening helps. As for eye contact, only he knows whether he wants you to look away or not. So when new people do not know how to react to his stutter, they will look around and then see you nodding and listening, and then they will resume listening to him, and they may even start nodding to copy you. So that is the good effect your nodding will have on new people.