My son says we keep hearing this same story over and over and he says it is as simple as 2 plus 2 is 4, yet some teachers just don't get it. He says your son needs to do academics at his level and be allowed to do writing at his level, with help for the writing disability and if the school won't allow this, then you will need to homeschool or afterschool. He says if parents of twice exceptional children want their children to be successful then they should fight the school for an appropriate education. He says holding a child back to the level of disability makes no sense. He said it doesn't make any more sense than placing a strong, physically gifted 6th grader in 9th grade based only on his level of motor skills. You have to teach to all of his abilities, not just the academic and not just the physical. He said when he was in Kindergarten it seemed like they expected everyone to be at the same level and he knew that he wasn't like the other kids. He would have loved to participate in a group discussion over thought provoking books with the older kids he felt more comfortable with, but that is not allowed here, so he does community musical theatre with older, gifted kids who are his best friends.
My son does not have selective mutism, but I think I did as a child. I know that I learned so much more when I felt accepted by the teacher both for my above grade level ability in reading and my extreme shyness. Their acknowledgment of my difference in reading ability gave me a boost in self confidence and I did well. I can't imagine what it would have been like if they had tried to hold me back for something like handwriting issues that I didn't have, but my son does.
I think your plan for the summer is good. I have my son practice some handwriting even in the summer, but all he will do in his own handwriting is copy work or writing spelling words. For difficult spelling words though, I noticed that he would say the word out loud first and then write. I have never been able to get him to write about anything on his own when he is using his own handwriting because it does seem to take up so much physical and mental energy just to write and he still needs lines to write legibly. He has to type to get his thoughts on paper.