You are most likely to win an appeal on the basis of:
1. your alternative academic evidence
2. any extenuating circumstances
3. reasons for wanting a place (if the school is now oversubscribed)
My question is wether to raise the issue of a subjective entrance qualification proceedure
I do think you should raise this, although panels tend to get uneasy when 'the system' is being challenged. If the system is flawed, it could be argued that everyone's result is invalid, and it doesn't follow that your son gets a place.
After any introduction, the appeal will begin with a representative of the authority/school presenting their case. You will then be asked whether you have any questions. This would be an ideal opportunity to raise your concerns. It's best done courteously, in a low key, non-confrontational way - a request for information rather than a challenge! You could also ask for a breakdown of your son's scores (if you still haven't got them). The panel are under a duty to consider the admission arrangements, so I suggest you politely ask them to consider during their deliberations whether these arrangements are objective and lawful.
I think they are entitled to change the format of the test, and it could be argued that this is fair because the test is therefore more 'tutor proof'.
Test papers are an exemption under the Data Protection Act, and you are not entitled to a copy, although the school could at its own discretion allow you to view them.
I see no reason why you shouldn't ask the school for a breakdown of your son's scores to help you prepare for an appeal. If they refuse, put the request in writing, say that it is being made "under the Data Protection Act", and send it by recorded delivery.