It sounds as if the family expects a bit more because you have raised their expectations. That is quite a success for you. Autism is essentially a communication difficulty as far as I know. (I used to work in a hospital for the mentally handicapped). As you have been successful with the child up to now, you will probably want to reinclude the child in your class. You will probably only feel able to do that if you establish better lines of communication with the family. That will rule out suing, calling the union or trying to shift the student.
Obviously, your line of communication to the family will not be improved by the eighteen year old's tantrum. You will probably make it clear to the family that you want an apology and that they have to see that the needs of all children and the teacher have to be taken into account. It is the child who lost control in the situation - not you. I am sure you do a very good job.