Welcome to HealthcareMagic.
The medically related reasons for his bad performance in studies can be
1)some youngsters have thought disorders and learning difficulties that even they don’t know.This makes them difficult to learn and so they avoid it.
.These days drug abuse has become very common in teenagers and that remains as an addiction.When a person is addicted to a substance or habit , it is difficult for him to concentrate on other things
Please observe your brother and try to find if he has any difficulty in keeping his academics upto his level.
At this age , the development of brain occurs in a way to be very flexible.The ideas and perspective towards his life that he gets now remains firm after 20s .So it is important he has a focus on what he really wants.
Though it is difficult , it is possible to change a teenager`s behavior than a person of 20 s.that’s evident by scientific studies too.
I guess that helps you and all.
If you need any help you can ask me.