Wow. I'm sorry to hear that it's such a long process to get your son the special accommodations he needs. It's a shorter process here - still a bit involved, but it can be done in a matter of a couple of months. My daughter's special education plan moved with her to her new school when we moved across the country 3 years ago. The new school still needed to adapt the plan for the services here, but it wasn't a long qualification process. That's awful that they make him stay late to finish writing instead of giving him the technical supports.

If my daughter is having a hard time with writing a lot of things, she has the option of dictating to her classroom aide who will then write for her.
Definitely try to get him that piano time. It may make the difference in getting him through the next year. When will he be seeing the doctor? How much longer for the school year for him before summer holidays? My girls only have about a week left of school, then vacation until the end of August. My younger daughter, who will be 10 on Wednesday, will be moving up to the middle school (grades 5-8) next year, so she's pretty excited about that. My 13 yr old will be in her last year of middle school, so she can sort of help her sister get around a bit.