Could it be because he's scared @ChristopherTracy
Mine is a bit like that, easily overwhelmed, lacks confidence, reluctant to try hard in case he doesn't succeed, silently panicking inside. It's very tough to deal with or it is for us anyway. DS did a few papers at the end of Y10 so we had some practice last year.
Can you get revision books if he has revision notes missing? My DS's notes are so poor in some subjects that he has abandoned them altogether and is basically working from revision guides and past papers.
Talking definitely helped us - I think the thing that eventually made the difference was getting him to think about how he wanted to feel about the exams and himself. He agreed that doing badly because the paper is really hard is ok, but doing badly because you haven't worked hard enough is a horrible feeling to have.
I kept saying, I don't care what grades you get, this is not for my benefit, but I want you to feel good about it, to go into the exams feeling confident, to know that you did your best whatever happens.
You are probably doing all of this. In which case I can only say keep going. There is loads of time before proper exams, you have got time to turn him around. (You will feel awful too if you give up and he does badly!)