Ok, so here's the history:
Parents are animal lovers with soft hearts. They have 13 dogs and 30 cats which all get along together most of the time (usually they have no problems).
Bruno (Pit bull/Boxer mix) was found by my parents 4 or 5 years ago. He loved people but didn't get along with other dogs. He ended up attacking a smaller dog right away and my parents believed that because they took him on as their responsibility they couldn't really get rid of him, so they decided to keep him separate from all the other dogs. Outside he had his own 7 ft fence inside the bigger fence where the rest of the 'outside' dogs were and was crated at night, straight from outside. My parents noticed that he got along with another dog so they started letting the two run together at night when they had brought all the other dogs inside. Well one night they forgot one of them in the cage and let the two run loose. They ganged up on the forgotten one and gave him some bad gashes that needed stitches.
Jump forward a year or so.
I was in my senior year of college when I took the dog to school with me. My roommate also decided to bring his Poodle mix down. Poodle mix attacked Bruno as soon as they met so we didn't leave them alone together until one night when we forgot that they were loose and stepped outside for a smoke. When we came back in Bruno had popped the eye out of the poodle mix. Poodle lost the eye but was otherwise unscathed. My roommate made me send my dog back to my parents where he resumed his life as described above.
Jump forward two years.
Bruno has attacked two or three other dogs so my sister takes him as she only has one dog. I get laid off and move in w/my sister with my two small dogs. We kept them separate for a while, started walking them together and they seemed to get along well so we started allowing them to be together while supervised. We had two or three altercations where the smaller dogs received small puncture wounds on the tops of their heads which we were told by a trainer that this would indicate that he's not trying to kill them, but assert his dominance. The trainer told us that leaving the little dogs out and crating the bigger dog was confusing the dogs. So we started crating the two dogs in one cage and Bruno in his own cage.
Three days ago my sister forgot to crate them when she left and the small dog was attacked, but he only had two small dots on his head. The 'altercations' as we like to call them are getting less and less severe. However, today Bruno and a cat got in to a fight (which has never happened before, Bruno usually licks the cats and tries to cuddle with them) and the cat came out with... you guessed it - a ruptured eyeball.
I don't know what I'm doing wrong. I do obedience training and he listens to me. We've used treats to reinforce "sit", "stay", "down", "go on" (our version of leave me alone) and "wait" (to wait to come in from the supervised back yard trips).
My family and I have talked about putting him down - he's attacked a lot of animals and we don't know any of his history before we got him. I've never actually seen the fight start, it just happens right around the corner or when my back is turned, so all I can do is stop the fight. I don't know what's setting off the trigger, but the cat thing that happened today is out of the ordinary (if you consider 10 - 12 different fights with other dogs ordinary). I've had to yank him off of my little dogs a couple of times but since the obedience training he will stop fighting now when I scream "Hey" and then tell him to "come here" - he comes right to me and leaves his 'prey' alone. I don't hit him when he comes to me, but I've had to wallop him a couple of times in the beginning because I was afraid he was killing the other dog and couldn't get him to stop (learned about the wheelbarrow trick just for this reason). I can't figure out what keeps setting him off so I don't know what behavior needs to be corrected.
Sorry if this was a little winded but I thought if you all had the history as I know it, you could point me to the things I'm doing wrong so I can correct them and don't have to put the dog down. A few other things - he gets extremely overexcited when strange dogs come around and he's definitely got a lot of energy, which we try to dispel with games of fetch and walks around the park, but even though he has become very well behaved towards humans, he's still attacking smaller animals.