shockedandupset, I think you've had a very hard time and hackmum has probably put her finger on the reason. A lot of people here seem to be thinking 'I did something different - I am always right - therefore she is wrong'. Not edifying.
Of course you're upset. You are a graduate and you have taken it for granted that your son will follow in your footsteps. You know he is capable of it. You have assumed, not unreasonably, that even if he is at a less than brilliant school, he would still do well. You see that view expressed here all the time. Home background is the single biggest determinant of how well somebody does in the education system. So he has that on his side.
Now you have to work out how to support him in his work to get where he wants to be. To get into a good sixth form, he needs good GCSE grades. Ignore those telling you that in their area you can go on to do A levels with all Bs. You have checked, and in your area he needs As and A*s in the A level subjects and Bs in the other core subjects.
One thing to bear in mind is that if his current school is not very good, when he applies to university that should be taken into account. Universities are very keen to widen access and they should be using contextual information to help them spot applicants whose results are lower but who nevertheless did well when you take into account the type of school they were in. I think more weight would be given to this for A level than GCSE, but it might help a bit, nevertheless. Worth checking out, perhaps.
I would second those who are recommending getting an experienced teacher as a tutor. As far as I can make out, nowadays a lot of the trick of getting top grades is having a really good exam technique as much as having good subject knowledge. This is rather depressing but you have to play the system as it is now. An experienced teacher should be able to help a lot with that. I'd also agree that going on an intensive revision course at Easter would help.
I think it's far from unusual for boys, even when they're quite clever, to get a rude shock in year 11 when they finally grasp that they do have to revise for exams to get through them. Take comfort from that, and good luck!