one of my good friends used to be shia and after debating with him a few times alhamdulilah he become of ahlu sunnah..
There was a webstie ( i will search for it for you inshaAllah, but i can't now) that showed so many errors from them, when he saw that alhamdulillah he eventually gave in.

But guidance is in the hands of Allah, so the best thing you can do is make sincere dua for him.