Hello Noella, many thanks for posting your comment and understand the situation you both are in, especially suffering from any emotional and constant criticism by his mother, which may indeed flow onto you as well, this denies any opportunity of being able to form any confidence within yourself and I'm not a doctor qualified to say this.
The help you are trying to provide for your brother is monumental and be somewhere he can cry on your shoulder to explain his frustration, while perhaps you are hiding your own problems which are also a great concern.
I am sorry that your mother is inflicting this obscene pressure as it doesn't allow anyone to develop their future, with friendship, careers and even trying to enjoy any hobbies.
Sometimes a sibling trying to help their brother/sister isn't overbearing, it's a place for them to come and express how they are struggling and not seen as controlling.
If I can suggest that he contact Kids Helpline 1800 55 1800 by phone, webchat or online for someone to talk to, and these are trained counsellors, but adapt to the age of the person they're talking with.
Another option is, are you both in a position you can move out of this home and there are many possibilities this can happen, but wait until we hear back from you.