I just wanted to give you the website to another awesome dog trainer, who actually works with Ceasar sometimes(who I absolutely love!!! even named one of my pets after him....LOL). The trainer's name is Ed Frawley and he deals alot with German Shephards and training for schutzhund trials and police service training. He has a really awesome website and I have used some of his training techniques on my Lab (who was similar to your boxer... but I my lab had more dominance type issues and was always trying to dominate over my older dog who is much more submissive). Now some people don't like him because at first his training methods seem a little harsh, but it combines positive rewarding with corrections. I know that you are going through a tough situation and I wish you the best and hope that you can find your dog a great home!! But just in case you haven't fully decided to rehome him yet, this is the website:

http://www.leerburg.com