All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, we ask Allaah to have mercy upon your brother and make his disease a means for wiping out his sins and raising his degrees. The Prophet said: “An affliction continues to affect a believing man or a believing woman in his/her own self, his/her children, and his/her wealth, until he/she meets Allaah without a sin.” [Muslim, Ahmad and At-Tirmithi]
It appears that that man you mentioned is a magician and a liar. In general, such people claim to know and inform about the unseen, and such an act leads to dissension between people and causes separation between those who love each other, and this act is beloved by the devil. Therefore, it is not permissible in Islam to go to magicians and soothsayers as we clarified in Fataawa 89807 and 18831.
Moreover, no one who fears Allaah would claim such an allegation that is mentioned by that man, which is his claim that the mother did magic on her son. Whereas, in principle, a Muslim is innocent and it is not permissible to accuse him/her of doing magic to anyone without evidence. No doubt, a mother is more innocent as, in general, she is affectionate and loving to her children and she endeavors to achieve what is beneficial for them and repel harm off them.
Allaah Knows best.