When he is nips at these people are they running, moving fast, are they afraid of him? When he does this I would take him, by the collar and put him in a time out in his crate. After 5 minutes take him back out and if he gets excited to see them and he starts mouthing them again, back in the crate for time out again. Do not give any emotion, or words to him, have his crate in the room and as soon as you see the signs he is over his threshhold, in the crate he goes for a time out.Without seeing him in action tt does not sound like aggression to me, just over stimulated and he gets mouthy but you are right in addressing it because it could lead to a bite. Also you may find helpful the book called Control Unleashed, you can get it at Amazon.com or Cleanrun.com. It is ideal for a dog like him that needs to learn some self control. There are some games they teach in it that would be very helpful.
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