Congratulations! Bruce has stepped up to the plate and accepted responsibility of being the Alpha dog, since no one else would do it!
The professional trainer is absolutely correct in her/his assessment of how to correct the behavior. This website will explain it better to you and why you need to get control.
http://k9deb.com/socialis.htm The other site listed above is on the same site but the one I listed should be read 1st and put into practice 1st.
You are really doing a disservice to your dog, by making him the alpha. He has too much responsibility (big task) for such a small dog. He is buckling from the strain of it and confused on top of it all. Once you take the burden from him, he will be relieved and start to relax and no longer have these aggression issues.