Thank you, everyone for your helpful suggestions. As far I as I know, he has not shown the aggressive-dominance behavior with adults. Aside from our son, he is not around children. I am not completely sure of the mix of breeds in the dog in question. He is black with the rather "splashed" markings of a border collie. On his muzzle he has black spots similar to those of a springer spaniel. He also has the rather "fluffy" ears of spaniel. He is long, but low-slung, with a barrel chest and smaller hind quarters. Also, his hearing is not 100%. We had taken him to obediance training and he was a "B+" student. We've tried obediance training with the female, but she just couldn't take the change in atmosphere -- the other dogs, the other voices, the commands, etc., so I have been working lots of hours with her at home and she can do the basics. You've no idea how long it took just to get her to wag her tail and learn that it was o.k. to play! The vet. tells me that there is nothing physically wrong with the aggressor dog, so I will try returning to obediance classes with him as well as consulting with an animal behaviorist. In the mean time, I will keep my son and this dog FAR APART! I hope that I do not have to find him another home, but if I must then I must. Thanks again, everyone, for your helpful suggestions and kind words. I have always enjoyed this site (from my desk at work !)