I was predicted all A's in every subject, but due to declining mental health has a result of OCD, depression, and being severely bullied i dropped out dead on half way through year 10. I spent the next year with no schooling what so ever, then finally about 3 months prior to the GCSE exams i started getting 45 minutes a week home tutoring by the local hospital of english, 45 minutes of maths, and two 45 minute tutorings of science. I also got 45 minutes a week of elective history. I had a year and a halfs worth of class work, homework, and every single piece of coursework to do, in the space of 3 months. It took doing 3 english pieces of coursework in one weekend to get them to relent to let me take both english literature and english language.
I desperately wanted to take the higher level papers, they let me take intermediate maths which allowed for a maximum B grade, and because i had only covered the topics of one of the 2 history papers i took only one of the two history exams which is the equivalent of half a GCSE, which wasn't tiered so could get any grade from A to fail, but insisted i could only take the foundation level papers in both English courses and Science.(this was in 2006 so science was done as one subject GCSE, not split into chemistry, physics and biology seperately)
No amount of pleading would change their mind. They knew intelligence wise i was capable of the higher papers, BUT because of how little time i had to cover so much of the curriculum and because they could only teach me for less than 1/3 of the time i'd get per week in a classroom face to face, they just wanted me to be able to get a passing grade, and it is easier to achieve a C on foundation paper, than it is to achieve a C on the higher level paper.
As it goes, i achieved a B in maths, a B half GCSE in history, and C's in Science, English literature and English language. Looking back now it was pretty massive an achievement to get the grades i did in my circumstances, and they were enough to get accepted onto the college course i wanted to do, but my illness continued to get worse so college wasn't a possibility anyway.
The fact is, your son only got a C in his mocks, he was predicted and A so the difference levels him out at a projected B, but it is more likely he will only get a C. The C is easier to get on the foundation paper, and a pass is more important to the school for their pass rate tables than your sons future prospects. It is incredibly wrong, but the school are more interested in their own agenda, to get him to pass.