Hermanhessescat, whilst all these arguments are valid, I feel the use of a scribe has been overlooked. It is much more difficult to get a good grade in English Language if you use a scribe compared with other subjects because of the emphasis on marks for spelling, punctuation and grammar. As an example, in AQA there are two writing questions which both have 16/40 marks allocated to SPAG. To be eligible for any marks for spelling you must spell out every word, letter by letter and for punctuation every full stop, capital letter, comma, setting out of speech etc..must be dictated by the candidate. It is really hard for someone with a scribe to do all this without losing their flow and train of thought and consequently the marks for content can also be affected .
With AQA there is a ladder for marking the writing section which read literally means that someone who doesn't spell out letter by letter every word cannot get off the bottom rung, even if their grammar and punctuation are good. We sought clarification of this from the exam board but the answer was rather non-committal.
So, if there was a difference in the way your son's teacher applied the mark scheme in the mocks compared with the the examiner, that could explain the 'drop' in mark. On the other hand, was the exam marker clear on how to apply the mark scheme for someone who had a scribe? If not, there may have been an error of judgement and a review may pick this up. If you ask the school for the marks in the individual papers it may show up an anomaly in one paper, in which case it may be worth requesting a review, but to get both papers looked at is costly and it sounds to me as if your son would prefer to do the retake and try to get a 5, so if it were my son I would save the money and encourage him to do this. In the meantime, I would get his new school to find out how the exam board applies the mark scheme for those with scribes to make sure he (and the scribe) understand what he needs to do to pick up as many marks as possible in the writing section when he retakes the exam.