However, he's not working up to his potential -- everything takes too long for him. If no one is going to listen, he'll just give up when the work load becomes too heavy. He's so talented that I would hate for this to happen. What else can I do with the school system?
You stated that your son had a helpful 504 plan in elementary school. Was it renewed by the school district when he went to middle school? Plans are typically reviewed annually. If his current school testing shows that your son has a significant discrepancy between his ability and his school performance, then he is entitled for the services to continue. The school counselor might be able to help him accept the services he is entitled to, and to see that help is not delivered in such a way as to embarrass him in the classroom.
You said that you think your son has dysgraphia. Webster's Dictionary defines this as an inability to write adequately due to brain damage. The Diagnostic Statistical Manual 4th Edition (the DSM IV -- the sourcebook for mental health diagnoses) identifies a "Disorder of Written Expression" as "writing skills (that) ...are substantially below those expected given the person's ...age, measured intelligence, and age-appropriate education".
A diagnosis is required for a 504 plan to be put into place, and individually administered standardized tests are needed to determine if there is a learning discrepancy. If your son has an active 504 plan, then they are usually required to test him every few years. You should ask the school if they will accept outside, private, educational testing, which you may need to obtain. Your son may be eligible for interventions such as shortened written assignments, the use of a computer for written work, etc.
Don't hesitate to be an advocate for your son and his learning needs. Older students do indeed often give up when they get too frustrated. If the goals on his educational plan are not being adhered to, I would encourage you to contact your state's Department of Education and ask to speak to an omsbudsman -- a problem solver -- who may be able to help you further. Good luck.