Praise be to Allah.
People who have had experience with such situations have related that the following are among the signs of a person who is possessed by jinn (or Satan):
Strong repulsion when hearing Quraan or Aathaan (call for prayers).
Episodes of losing consciousness and/or epileptic attacks, especially when Quraan is recited for the possessed person.
Frequent nightmares during sleep.
Tendency to avoid people accompanied by out-of-the-norm behavior.
The jinn who possesses him might speak when Quraan is recited for the possessed person.
Madness, as stated in the Quraan (interpretation of the meaning): "Those who devour usury will not stand except as stands one whom Satan by his touch hath driven to [epileptic] madness"2:275
As for a person struck by magic he might experience the following:
Dislike of ones spouse, as indicated in the Quraan by the following verse (interpretation of the meaning): "And from these (angels) people learn that by which they cause separation between a man and his wife..." (Al-Baqarah, 2:102).
Different attitude in the house from that which is outside the house. For example, a person will feel that he is missing his family when is outside the house but when he goes home, love changes quickly to extreme hatred.
Inability to have sexual intercourse with ones spouse.
Frequent miscarriage for pregnant women.
Sudden change in behavior without obvious reason.
Complete loss of appetite for food.
Thinking or imagining one has done something when in reality one has not.
Sudden obedience and/or love for a particular person.
It should be noted that if a person experiences some of the above symptoms this does not necessarily mean that he is either possessed by a jinn or struck by black magic. It might be due to physiological or psychological reasons.
As for curing this condition the following steps are recommended:
Putting ones trust in Allah with sincere belief that He is the only cure for everything.
Reading Quraan and known supplications expressing seeking refuge, the most important and effective of which is sura 113 and 114, Al-Falaq and Al-Naas, which were used to cure the Prophet himself. Surah 112, Al-Ikhlaas, is recommended along with them, as well as the opening chapter of the Quraan, Al-Fatihah. To cure black magic some have successfully used seven lote-tree leaves. The leaves should be crushed, then mixed them with water enough for taking a bath. The following verses from the Quraan are then recited: verse Al-Kursi (2:255), surah Al-Kafiroon (109), surah 112, 113, 114; the verses which mention magic, which are: in surah Al-Baqarah (2:102), Al-Araaf (7:117-119), Yunus (10:79-82), and Taha (20:65-69). The possessed person drinks some of the water, and the rest is used to give him a bath.
Removing the elements of magic as was done by the Prophet when he was struck by black magic by a Jewish man called Lubaid Ben Al-Aasim.
Eating seven Aaliya Al-Barniy dates (among the dates of Al-Madinah) first thing in the morning; if not possible, any dates will suffice, by the will of Allaah.
Cupping--removing excess blood.
And we ask Allaah to cure your brother and ease your hardship and his, as He is the One who cures and there is no one else who can cure.