It can be tricky to figure out what your child's writing level is, since standardized testing often doesn't work well for children with autism. I find it helpful to get a sample of your child's writing and compare it to samples written by children at different grade levels. You'll find the writing samples that your child's writing most closely resembles, and use that a grade-level estimate of your child's current writing skills.
A great resource for 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. If your child has the language skills, you can give him or her a picture and a blank paper and ask him or her to write a story about the picture. If this is currently too difficult for your child, you can 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's a graphic organizer 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.