This is a bit of a weird one and I'm not sure how best to handle it. DS is a high achieving very driven student in an average comp. He's head boy, only one to get all 9s at GCSE, is predicted 3 A stars, got that in his mocks and has an Oxbridge offer. I'm not bragging but obviously this is relevant to the post. He absolutely loves school and his teachers and parents' evenings are always glowing - as much in terms of his attitude as his achievements. He is always being praised for his contributions to discussions (does hums/arts subjects) and his questions as well as the way he asks for and responds to feedback. It has never so much as been hinted at that he is arrogant and I really don't think he is. He has a wide range of friends and helps out at revision sessions for younger pupils, open evenings etc.
Lately he has been commenting that in one subject he is being marked down by teachers and disagreed with in class, he feels, for the sake of it. To give an example, essays are marked out of 25 and he has scored between 22-25 throughout the course, usually only dropping a mark or two for the last year. However, now he is getting 21 or 22 most of the time and it's knocking his confidence. As the final exams approach it seems he is getting slightly worse which is worrying him. This is the subject he is doing for a degree as well. He also feels the teachers are disagreeing with him a lot in class and feels like he is being 'put in his place' so to speak, and he finds it hurtful, especially for it to start quite suddenly at this point.
I am trying to put it in perspective for him and saying they just expect so much from him and want to be sure they have done everything they can to help him meet his offer (he doesn't actually need A stars, but he'll feel a failure if he doesn't get them, especially in this subject) and that could be why they seem 'picky'. The discussion-based stuff could just be a perception, which I have said to him. I realise this probably sounds ridiculous but it's getting to him and it's not nice to see. He has loved school but puts so much pressure on himself and now he's not sure what's going on. What, if anything, should I do?