C.J.
That sounds just like my husband! My husband was a lot like that when he was that age. He nearly failed school because of his lack of caring, lack of respect, and because of his constant fighting. He was ALWAYS in trouble! Anyway, he started martial arts. It taught him discipline and respect, not only for others, but for himself. It gave him an environment where he could positively direct his anger. He did a complete 180! As a former martial artist myself, I can definitely tell you that it is an attitude changing experience. You'll be amazed at the transformation. And once he gets into it and starts competing, he will be proud of his accomplishments and you can show him how proud you are too, which will help his esteem (probably the underlying cause of his aggression). Try it for a while, no less than six months so that he has a chance to really get into it.
I also agree that your son may feel unwanted or unloved. Try to include him in things. Take some time for just the two of you. Maybe he will open up and tell you what is really going on. Provide stability. Do not send him away again. That is only a last resort. There are many more options for you to try first. As in my husband's case, he has ADD and his mother treated him differently because of it. He was hurt because she chose to spend all her time (and money) on his brother. He lashed out just to get her attention! His dad was a salesman on the road all the time, so he wasn't much help. He was frustrated in school because the teachers wouldn't help him learn. Kids made fun of him because he wore glasses, etc. There is something in your son's life that is eating at him. And no disrespect intended, but it's probably his parents. But try the martial arts thing. It'll be good for him physically and mentally. Good luck and best wishes. I hope it all works out.