Actually quite a few students want to repeat F6 EVERY YEAR. School has only one or two quota even though all of them have very strong affection for SFXC.
MR Yiu has been very patient in answering your enqiries! And he has tried his very best in helping your son!
Your feeling can be understandable but the school actually can do very little as the education resourse is scarce and the school has given your son a chance before. If your son is allowed to repeat F6, that will be unfair to those who cannot repeat in sfxc and similarly love sfxc as you do.