Praise be to Allah.
Yes, what your brother is going through, whether it is a physical disease or sihr (witchcraft), or it is a case of mental illness, or anything else, is indeed a test from Allah, may He be exalted, to His slave. Every test that a person goes through in this life happens by the decree of Allah, and it is good that your brother realises and understands this.
But what is not good and is not correct is for him to think that he has to wait until Allah, may He be glorified and exalted, lifts this suffering from him. Belief in the divine will and decree does not mean that one should not take appropriate measures. Rather, when Allah, may He be exalted, decrees something He also decrees the measures that could lead to it; feeling full happens by the decree of Allah but the hungry person should eat (to reach the state of feeling full); and having children comes by the decree of Allah, but the one who wants to have children should get married, and so on.
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Allah has not sent down any disease but He has sent down a cure for it.”Narrated by al-Bukhari, 5678.
Thus it becomes clear that as Allah decrees illnesses, He has decreed remedies for them that are appropriate; those who know them know them and those who do not know them do not know them.
So what you have to do is to treat your brother kindly according to his situation, and convince him to seek a remedy for his sickness. It is good to use both ways together, the usual medical remedy for the one who is in such a situation and looking into the matter of sihr, which is by his going to a raqi (one who recites legislated incantations) who can treat him with ruqya by reciting Quran and proper, well known shar‘i (Islamically legislated) ruqyahs (incantations).
We ask Allah to heal your brother for you and to help you in dealing with him, and to give you the best of rewards.
And Allah knows best.