First it is important to tell your parents because my suggestion is you should not enter into this directly yourself as it would cause problems for you. Your parents will have to deal with this calmly so as to maintain a strong bond with your brother.
Secondly, your brother will need to be referred to a counselor/psychologist to help him stop watching porn as this can be quite addictive and requires professional help to leave. If there are any other problems he is facing that are the root cause of his actions, it can be solved by a counselor as well.