Try main streaming your child. Take in your paperwork stating Autistic. With this you and put in a written letter that you want your child main streamed. They must by law respond to you within 30 days.
If this doesn't help contact RESA and request an advocate for your child. Most schools will hop thru hoops of fire, before dealing with RESA

That is absolutely outrageous! I am so sorry about all these battles that must be waged in advocacy for our children. It's a huge violation of an important federal law Public Law 94-142 that all children must be educated in the LEAST restricted environment and IDEA (Individuals with Disabilities Education Act). If a school is found to no be in compliance, they will lose their federal funding. You really should contact your state's senator's office and tell them this issue. Once the school gets a call, or a formal complaint through that office, it's extremely likely they will comply.

I challenged the school district when they refused to place us in a general classroom with an aide. It is technically a violation of FAPE. Kept mine in private Preschool with an ABA aide. We have no ABA centers around here. Keep him where he is doing well. If it’s social skills with peers that your child needs to work on, it makes little sense to place him in an environment where all the other kids are working on those same goals. Some districts have mixed classrooms and I have heard good things about those. Even in the private school, I am having to keep an eye out to remind them not to segregate the children who need extra help.