Advice on how to support ASD teenager and exam anxieties

I am after some advice as to how best to support my son who has ASD and is really struggling with anxiety and stress, often brought on by school and exam pressure.

He has recently started his A Levels and each time he is asked to take a test to see how he is doing he isn’t writing anything as he is too worried about the possibility of getting a bad mark so would rather not answer as failing by this way was expected.

When he has tried and he hasn’t got the mark he was hoping for he becomes so upset and frustrated that he has started to break things at home as a means to releasing his emotions. And this then leads to him becoming more upset and worried as he feels awful for doing so. This week he pick up his glasses and smashed them to pieces.

We are really struggling to know how to support and help him with this. It’s not the issue of being upset that he has broken things as we know that at the time he can’t help it, but we would like to know if anyone else has gone through this and if anyone can offer and advice on how we might break this cycle as it is causing him even more distress.

Many thanks