Thank you so much for your information. I have been doing some thinking about the canine-human relationships in our home since reading your post. I had established myself as the "top dog," so perhaps the dog in question is challenging my position, or my son's position as the most-favored. We rescued this dog when he was a scarcely-weaned pup and had been in a terribly neglected state. The other dog, the one who is very passive and patient with my son, is a female and we rescued her at about the age of 6 months from a dreadfully abusive situation. She is the most submissive dog of our "pack" and has required many, many months of pateince and training to convince her to perform the simplest tasks. I live in a small town in southwest Kansas and my vet. at the moment does not know of an animal behaviorist in the area, although he is looking into it for me. He has suggested, as you have, that I keep my son and this newly-aggressive dog apart. Unfortunately, I am already considering finding another home for him, one without small children or other animals. Again, thanks for your information and advice.