Nikki, we have 2 girls of our own and currently fostering a kindergartener who pretty much sounds like your son. He came to us with no respect to authority, not knowing boundaries, throwing fits if he did not get what he wanted, etc....His first day at our daughters' school I got a call from the counselor saying he would be reported for threatening to beat up and kill some of his classmates.
Well, we were beside ourselves when this behavior did not stop after doing exactly what you tried and nothing worked, we decided on "tough love" after 2 weeks of unbearable behavior because we knew there was a child underneath it all who was so capable of love and good behavior. Now I know this will sound harsh, but basically we "disconnected with him". When he kept coming home with a signature from his teacher, we explained why we were punishing him and he basically had to go to his room after school (we removed all toys from it) for about 30-60 minutes so he could "think" about his actions. Then we had him sit down where I was working (cooking dinner, helping my daughter with homework, etc) and he was not allowed to do anything but talk to me, most of the time it was ignoring his plea to get up and do something. After that it was writing apology letters to whoever he was disrespectful to at school. Then it was dinner with no tv, no video games, no park, no scooter, no board games, NOTHING afterwards. Dinner normally ends around 6:30 and he went straight to brush his teeth and to bed. The first day he threw the biggest fit that we could hear him from the front sidewalk of our house. The second time the "fit" lasted about half the time, then by the third time he only shed tears and apologize for his misbehavior. After the 3rd day the teacher saw such a difference that she asked what we were doing and I just told her good old fashion discipline. Our foster son now brings stickers for good behavior most everyday. He will have a set back maybe once every 1-2 weeks but it is nowhere near the type of behavior when we first got him. BUT, because we do not want the old behavior back we pretty much discipline him the same way when a signature is brought home for misbehavior. When he brings home a sticker for good behavior we constantly remind him that he made the choice to be good and has the "privilege" to do and play anything he wants and be a part of our family time in the evening. He now calls us mom and dad and we are starting to really enjoy him. He is not perfect, but has come such a long way considering his biological family situation of drugs, abuse, and neglect and is such bright star in our family now. Dad calls him his "little hero" and we love him very much!