A lot depends on how the student is acting out and his behavior patterns. Is it a sudden change in his behavior? What are the environmental factors (poor students interaction, has there been a room change, lighting, lack of sleep, diet, etc.) that may cause poor behavior? I assume you have talked with the parent(s) and viewed his IEP for clues and his past history. So, it would have to be a matter of setting some type of boundary he would find uncomfortable but yet forcing him to see the benefit of comforming to set classroom rules.
Sometimes giving a student a goal to work towards helps. (i.e., If you can do this for so many minutes then you can have this privilege.) I would consult with those who can give you some feedback and support in dealing with this student. Documentation is key. And, sometimes you have to talk about removal from the classroom if it is too disruptive. Hope you find a solution for this. I know how much this weighs on a teacher's mind and affects students.