Q-actually it happened 15 years before. it was 100% consensual. after that we had connection over phone ,what's app . I have some proof also that he sent to me. now my question is weather after 15 years he can make any false case against me. because he used to told me his father is powerful person. I never see his birth certificate . I only believe his word.
Answer- If this happened 15 years ago, then the period of limitation for filing a case has already elapsed and your cousin cannot take any action against you now,after the period of more than a decade.
So you should try to get evidence in form of audio or video recording when the boy tries to blackmail you again, and if he still persists, then you can approach the police and file of criminal complaint against them for blackmailing and threatening you.