It isn't just based on mock exam. You have to give an estimate of where you think the pupil is, and the names of the pupils who are directly above and below that estimate. Students often cock up in mock exams. The estimate is based on their level of ability as judged by the classroom teacher. I do sometimes used mock exam if needed, but genrally I make an overall assessment of what grade I think they would have acieved if they were to sit the real thing.
Your poor ds has enough to worry about. I have done loads of special considerations for candidates, and they end up with the grade they would have got anyway (or pretty near) Tell him not to worry, he will be fine with it. Phone the school about the English exam and see what they say. He will have completed some coursework in English, and that is all part of the consideration as well.
It is really nothing for him to worry about.
I think, but am not sure, he could resit the exams in November, or the next summer if nothing has changed. But the books they study change in English, so I think that would have to be done in November.
Your poor ds, hope he is OK, if he was one of my students, I would really like to tell him not to worry about it