Violent Behavior

I am currently working with a student that has severe behavior issues. He is in Kindergarten and has not been diagnosed-yet. He was adopted by a family that is wonderful but at their breaking point. A diagnosis of some kind would be helpful and they are working on it, but this takes time. I am not sure how to do to help this child and his family in meantime. This child has two sides that intermittently come out: sweet and violent. He can be very sweet and kind for about ten minutes and then the other side comes out. He will suddenly out-of-nowhere tell the class that he is going to “come back and tear them apart.” He also bites, kicks, punches, and throws things. He shows no remorse when he hits or hurts someone. He has a one-on-one aide that goes home with bruises almost daily. She cannot wear jewelry because he will break or use it to hurt her. We have tried positive reinforcement, behavior plans, school-home support, half-days with an at-home tutor, and social skills class. We are at our wits end. We are a very low-income, rural, school district that houses about 200 children. We do not have a school counselor, psychologist, or behavior specialist on hand. I would appreciate any suggestions to better serve this child and keep the other children safe in our school building.