How can I change my dog's bad habits?

My German Shepard is 4 years old and he had been scarred from where we lived before. The kids in the next backyard would throw sticks at him when he was in the yard, and our neighbours were all gansters (sorry, I'm not being racist, but I mean crypts and bloods) and they had a rottweiler named *****. I know.. Stupid stupid name. Anyway, they would always be out with the dog and my dog, Bear would always be scared of it. After we moved, and Bear grew up, he got VERY territorial. Every time someone comes near our house, rings the doorbell, or comes in, he tries to rip their heads off. He only listens to my dad because my dad was the first one he saw and he's technically his "master." But Bear is so loyal, he stays loyal to my dad even though my dad has a bad temper and kicks him a LOT, and yells at him, and slaps his face when he needs to take his anger out on something. One time he kicked him with his shoes on and I wasn't aware of it when I was petting him one day and he yelped when I accidentally pet his bruise on his ribs; it was that bad. Anyway, my brother and I trained him a bit and he got used to people coming over sometimes. But then my brother, sister and I went to visit our aunt and uncle out of the province for 2 weeks. My brother and I trained the dog hard, so we told our dad, "Take GOOD care of him, PLEASE." When we came back, Bear was even worse than before and my dad ruined his training habits because he didn't bother to take him on walks or anything. Now he won't listen to us and my mom wants to get rid of him. How can I help Bear? He's living a really crappy life and I want the best for him. Help, please! ):