Glad to hear that your school is so progressive - you are fortunate! AND glad to hear that you are working with a psychiatrist - the MD issue is covered.
For my "typical" daughter with concentration, working memory and processing speed issues, a 504 plan that includes extra time, graphic organizers, models of expectations and laptop have made a huge difference. She is still not a literary genius, but she is more confident and able to produce better written work with the above. We chose not to put her on an IEP because we felt that in HS, the peer grouping for remedial help would not be appropriate. Instead, we opted for the 504 and hired a tutor.
My gifted son with severe reading and writing disabilities, also has access to a laptop. His IEP provides for intensive remediation. His school uses the EmPower program
http://www.architectsforlearning.com/empower.html and explicit models and techniques for producing written work. Based on what you have posted about your son, I don't think this would be necessary. I would assume he has mastered proper writing conventions/mechanics etc - otherwise he would not have scored as high as he did.
My sense is that maybe his executive functioning issues from ADHD are the roadblock - not poor writing ability/skills. Given sufficient time and a process to plan and check his work, I bet he will be ok.
