he got loose with a dog we had recently adopted (an escape artist apparently) and he started nipping at our neighbor
Did Mack start nipping at the neighbor, or the escape artist you adopted?
I know when I take 'doodle' to the dog park, the dogs can be a little "mouthy". They aren't biting eachother. No agression, no blood, no anger. They look like big kids playing the way they did with their littermates. Is that what you are referring to?