pickle, that is awful. How stressful and unnecessary, for both you and DS.
I have been racking my brains trying to think what might help you here.
If I could afford it, in your shoes I would actually consider instructing a solicitor to write a strong letter, on the basis that the school is not meeting its duties under the Equality Act 2010.
I haven't quite worked out the argument in my head, but it would go something like this. You are deemed to be disabled under the Act because you have (or had) a form of cancer. DS is 'associated' with you, so he is also protected under the Act. His recent difficulties at school probably stem from your treatment & illness; the school has entirely failed to make reasonable adjustments or take these issues into account, and have therefore directly or indirectly discriminated against him on the grounds of your disability. Or something like that.
failing that, would your local Parent Partnership assist? They exist for children with SEN, (and forgive me but I have not seen any suggestion that your DS has additional needs), but part of their remit is to support parents in disputes with school, and it might be that if DS currently needs extra pastoral support, they might be able to work with you/him.
PP www.parentpartnership.org.uk/about-us/
Best of luck with everything.