him being autistic means i have to prepare him as much as i possibly can
You need to start with the basics. The sorts of things he will be asked to do will be:
Put on/take off his own coat
Change for pe by himself
Toilet and wash hands without assistance
Sit quietly and listen to the teacher
Put his hand up before offering any input
Share resources
Take turns
Play with other children
Feed himself lunch
Follow the class rules
Ask for things he needs
Do you think he will be able to manage all of that by himself?
Regarding reading. There will be some children who can read well and others that have never attempted it. The range is vast and schools staff are used to that.
If he can decode, the next step would be to see if he can comprehend.
This means answering literal questions from the text and also making some inference.
For example, if the story said that Ted put on his wellies, waterproof coat and took his umbrella, what was the weather like?
How would your son answer that question?