Other children are often seen as a hindrance by children with ASD, so having "no friends" isn't really a drama, as he quite possible chooses to play on his own. People with ASD struggle to understand "our" world, and other people are really quite complex and confusing beings who don't like listening to monologues on bus times 
You may be able to help him by working out what it is about school that he struggles with. Social stories are awesome at covering stuff like that; you could use one to help him understand what is expected when, eg:
All children go to school. School is a fun place where we can learn new things.
I would like to go to school because I am clever and I am good at learning new things.
At school, I have a teacher who helps me learn. Sometimes I don't understand what the teacher wants me to do. I may get into trouble which makes me upset.
When the teacher says "Carpet time" I get confused. She hasn't said my name and I don't know what to do. I find this frightening.
When the teacher says "Carpet time" I need to go to the big space between the tables in front of the teacher. I need to sit on the floor, on my bottom with my legs crossed. I need to face the teacher and not talk to any of the children.
If I need to say something, I need to put my hand up. When I am allowed to talk, the teacher will look at me and say "Yes?" or "[name]".
If I get it wrong, it is ok because the teacher will remind me. Mummy and Daddy will help me by reading this story with me. I am good at learning things at school. I am good at [examples].
HTH