Wow! There can be so many reasons. This does not make him abnormal. I only occasionally wanted to study in high school. In my first attempt at college I went from dean's list to #%^@ list and basically flunked out. I quit studying. All I can say from my own experiences is that I did not want to study. I am not sure I have yet accurately self-psycho-diagnosed.
In my opinion, assuming that a student does not have a mental or psychological condition, or a learning disability, all the other causes can be lumped together under the category of motivation. There can be a lack of positive motivation for those who do not see intrinsic value in learning. There can be negative motivation for those who fear failure - it is better to have not studied and flunked than to have studied and failed.
One way to switch your brother’s situation around is for your brother to find a mentor he likes and trusts, someone who can help him find his own motivation. Seldom will this person be a close relative. However, when is the last time you heard about a student seeking out a mentor? The mentor can be a teacher who sees his potential, who relates to him, who truly cares about him, and who wants to spend time mentoring him. Perhaps there is someone who fits the bill.