Hello. It is good that you trying to help him out.
However the information provided by you is too minimal. There are no symptoms mentioned or examples given as to how he is behaving in a different manner. Are there changed patterns of sleep, appetite, and extreme changes in mood etc.
Next Steps
Seek consult from a mental health practitioner - psychiatrist
Health Tips
Try to keep a close eye on the brother and note down and behaviour that strikes you as strange