With the proviso that somebody has to come last, whatever their circumstances (and some will have benefitted from not having an exam to fail, be too anxious or ill to take or not turn up to), all schools have procedures for starting the appeal process.
Usually, it is the student (due to their age) contacting the school in writing or by email, requesting that the appeal process is started and in your DS's case, stating that their mocks were x grade, their attendance/effort had improved, the PPEs didn't have the special arrangements that were expected at the time of the summer exams, etc. Now, some of those grades were actually a combination of PPE results and aspirational targets, rather than actual grades, but the process will make that clear.
Once the process has started, it shouldn't take too long for the school to see whether there should be an appeal submitted to the exam board and advise him either way, together with a copy of the Subject Access Report (the GDPR rules require it).
A friendly call to the school, asking who you need to email to start the appeals process off, would be the best way to start.