Well it's really hard to convince especially if that person is not open-minded to that kind of thing. The only way you can do is to briefly explain to him what is Bitcoin, how it works, how to earn Bitcoin, why should he enter the Bitcoin world, its benefits and advantages to an individual, and the like. Try to convince him that it's not a ponzi scheme. Show your proof (your BTC wallet and cash--outs) to him. And after doing so, it's up to him if he will change his mind. The lost is not in yours anyway.