First of all, you should never tell any one or his family that he is a victim of Jinn. That telling itself can be a sinful act from you because of many reasons; A) You doubt about Jinn is not a concrete evidence that he is a definite victim of Jinn. B) Telling him or his family that he is a victim of Jinn will usually harm his mental state and make him feel defeated, helpless victim and negative. 
Second point in such cases is to recite Quranic verses which help in protection, prevention as well as cure such as Ayatul Kursi, Sura Al-Falaq, Sura Al-Naas, Sura Al-Hamd and Sura Al-Tawheed and other Suras.

Reciting Quranic verses will help in any case whether he is been harmed by Jinn or not. It will help in protection and prevention as well as in cure.

Wassalam.