There is no particular du`a (supplication) for such a case, but one can generally ask Allah to guide whoever is found of rejecting the truth and show him the truth or the right path.

It is noteworthy that no one is impossible to be convinced of the truth. The problem is mostly that he cannot find the right answer for the doubts and questions he has. Thus, it is advisable that one takes him to a trustworthy scholar to discuss with him or give him authentic books that deal with the problems he has.