Getting started with middle school language arts
Depending on your child's skill level, they may still need to practice reading. If they are proficient, it's time to move on to more challenging material.
Choose a chapter book, short story, or novel that is at their level, and read it either independently or together. Look up some "book club" type questions online for the most popular books. Use the questions or vocabulary lists to prompt lively discussions. If reading is still troublesome, read aloud or use audio books free from the library, and practice reading skills separately from analysis. Look online for "middle school reading lists" for good ideas.
Middle school is a good time to get into grammar more specifically. Keep it to the basics, such as parts of speech, usage, word choice, quotations, etc. Pick and choose what lessons to do, maybe do grammar once a week. Choose a workbook such as Growing with Grammar, or an online grammar program (IXL.com). Learning to edit is more fun in the Editor in Chief books, which have "find the error" pages.
Middle schoolers ideally could be learning to put together some basic forms of writing such as a summary from multiple sources, narrative stories, opinion essay, etc. Many neurodiverse kids (lots!) have a great deal of trouble with this level of writing, so take it slow, and give lots of help.
Seeing the "big picture" enough to write about it is a difficult task for literal-minded, detail oriented ASD kids. It may be necessary to walk though the steps with every project for several years, and that's ok.
If they haven't learned to type yet, now is the time. It's an essential skill in the world today.
Many times just writing about a subject you are already studying in Science or Social Studies is a convenient way to work in writing practice. Start with some outlining, finding keywords, and building a few organized paragraphs. Get an idea of your child's strengths and challenges. Just noticing how an author organizes their material provides great examples for use later.
Writing is hard to teach! If you want to leave it up to a professional, there are many online writing classes in which the teacher reads student's work and gives feedback. Try Home2Teach, WriteAtHome, or Brave Writer.
For a more thorough list of resources, see the Curriculum Planning pages.