It can be tricky to assess the writing level of a student with autism, since standardized testing often doesn't work well for these students. I also don't find it particularly helpful to get too caught up in test scores and such, since they don't often reflect your child's unique gifts and strengths.
However, it can be helpful to know an estimation of your child or student's writing level to help you plan lessons and activities that will be appropriately challenging to help him or her grow. If you're searching for activities on sites like TPT, it may also help you to narrow your search by grade level.
A great resource to help with this is Reading Rockets. On this page, they have writing samples of students in preschool through third grade. You'll want to look at multiple samples from each grade level to see which ones your child's writing matches best with overall.
You may be wondering how to get a writing sample from your child. One option is to give him or her a picture and a blank paper and ask him or her to write a story about the picture or write what's happening in the picture.
Another idea is to have him or her fill in a story graphic organizer to get an idea of what he or she can come up with for a story.
Here are some graphic organizers you can use for a pre-assessment of your child's writing ability. You will want to have your child fill in the form without help. You can point out the description of each section and clarify these descriptions, but don't give your child examples or ideas. It feels weird not to provide help and prompts to help your child learn, but for now, you want to know what your child can do without help.
Whichever method you choose, you'll just want to make sure not to give your child any prompts or help with writing. If my students ask for help, I usually praise them for asking, but gently reassure them that whatever they can write is fine...it's not really a test. It will just help me know what they already know so I can plan what to teach them next.
Once you have an estimation of the grade-level for your child's current writing skills, you can select lesson plans and activities at this level to work on with your child to increase these skills.