Speech for the autistic

I have an 18 year old autistic son. He is eligible to remain in public school until he reaches 22 y.o. He tries to speak in full sentences and uses all his words but is >90% unintelligible to unfamiliar listeners. The public school speech teacher only wants to serve him on a consultation level with the self contained teacher implementing a communication board. My son wants to and thinks he is already communicating so is unwilling to use the pictures to show one word and one idea. The speech teacher had another speech therapist conduct a series of evaluations to prove he would not benefit from direct services. He states that my child is accessing an education and that public school services are only provided if the child is not accessing his education. My child is in special education with an IQ of 54. We have been getting him ready for adulthood for many years in and out of the classroom and communication is a very large part of transitioning to adult living. How can we convince the therapist and school district that direct bi-weekly therapy sessions are in order. I have hired a private speech therapist for the summer months for the past several years and she agrees that school based therapy necessary. His classroom self contained teacher is also in agreement and has stated so in meetings with us and the therapist. Please advise me, we have another meeting this Friday.