My step-brother cares more about AS Critical Thinking than me!

Usually, we've been very close since we were young. We always talk and play games together since we were young, even though we have different social groups at school, cos we go to different schools, and I'm 1 year older than him - we always have a nice stable relationship where we can just hang out at home ya know. But ever since the Christmas December mock for AS Critical Thinking, he's a changed person.

At first, he just talked less and spent time in his room doing the critical thinking section from BMAT books. But steadily he's been not talking to me at all!! Whenever I speak to him he's so angry and resentful, and I don't know why! He says he's aiming for a C (and maybe even a B!), but his mock exam says he's working around an E grade.

He aspires to do "Alcohol and Substance Misuse Work" at Accrington & Rossendale College, and he needs around 96 UCAS Tariff points! He's really stressing out I think, even though he doesn't say it and says he's alright, I know he's stressed underneath. Critical thinking is his only AS, and he's doing a BTEC in Pattern Cutting Level 1 (that's a hobby of his) and Public Services Diploma Level 2. He has two GCSEs, Business Studies with D and History with a C. I don't know how to help him, especially with me going off to uni next year it's concerning - I want to be able to help him out.

I really feel neglected and helpless at the same time, I wish he would at least speak to me, I feel like a prisoner in my own home. I'm confused and I don't know what to do, someone please HELP! Thanks guys.