how is he getting into bars at 18?
anyway, i'll assume he's got a job to pay for his beer. If he has a job and you don't, that could be the source of resentment, provided you don't pay rent to your parents i guess it could seem like he's out in the workforce while you're chillin' at home going to school.
So thats more than a few assumptions, but maybe thats why he gets angry?
hope y'all work it out
