Hi Harman - couldn't pass this by
It sounds awful and your DS should not be put in a position where he is getting all kinds of shit bashed out of him at a place where he is supposed to be going to be educated and be safe whilst doing so.
I would let your exA get on with making an appointment with the teacher and think it would be best if you could attend this together to present a united front to the school.
I would also draft a letter to the head teacher, stating what the issues are, that you are seeing the teacher together but given the teachers previous dismissiveness to you, are doubtful that it will be resolved. So ask her for an appointment to be made now for the day following your meeting with the teacher on the basis that you can cancel it if needs be.
The reasons for doing the letter to the Head at this stage is to make sure that the documentary process is started now so that there can be no quibbles at a later stage as to who knew what when and whether the severity of the bullying and the shameful attitude of the teacher was understood by all parties.
If the issue is not resolved with the teacher then attend the meeting with the headteacher and at the same time write a new letter to the Governors and so on - escalating things as you go on. The key should be to move through the stages very quickly - withdrawing him again as soon as another incident occurs and pushing it up the line again and again for as long as it takes.
To my mind the key thing to explore is what sanctions are being taken against the bullies. Are they being excluded from school - taught in isolation - denied break times? Are these actions consistent with the policies set out in the schools anti-bullying guidance?
Getting your GP on your side sounds like a very good plan too
You must be in bits over all this - keep your chin up!