This sounds so hard. The school he's at is not the right nurturing environment to help him grow and learn.
I've been through this twice with different children, both with high functioning autism.
Can you contact your local National Autistic society branch and see if they can guide you?
Also look into others schools that are geared up for Social, Emotional, Mental Health (SEMH).
Visiting schools and ask a lot of questions. Join Facebook groups that have parents on a similar journey and ask advice.
The thing that upsets me is that he is NOT being naughty or wilful. He's scared. He's wired differently and his experiences and the way he perceives things are not the same.
Staff should always be asking themselves this question first. Is this child acting out of fear? Then ask themselves how to adapt his environment to help him.
You as his mum will know this about him, you obviously do and you are tightly concerned. You are up against professionals who are meant to understand and help but just do not have the training often or the time.
The key thing is to get support for his autism and get a trained professional to come alongside you to help the school.
Another place of advice might be your local PRU. I'm not saying to send him, I'm saying call them and talk gather whatever support, ideas, names of people in other organisations that might help.
Another place to get information might be a well informed GP. All you need is a professional that understands him to support him.
Don't force him in. The mental damage this can set up can have lasting negative consequences.
Please link to a Facebook page called 'Not fine in school: family support for school refusal and attendance issues'
They offer sound advice around the legal implications of protecting your child and battling with the school.
The final point, this school is not supporting your son. They are not being inclusive, not making reasonable adjustments to help his learning.