I will tell you one thing, I don't condone disowning your family over this like some do, but I will tell you this, OP, if he's as far gone as he sounds and you really don't want him going farther, you have to draw a line in the sand. Tell him you may still love him and be there for him when he's really in need, but in general, you aren't going to be around people who spout these kind of conspiracy theories. And you have to be willing to do it, too.
People that deep into the hole are basically at a point where they are just looking for excuses to fall deeper. They have to know there's a real consquence to thier own personal lives to snap them out of it. To make them realize that, hey, they are in reality, not some fucking movie where illumainti are staging planned school shootings for no reason.
You should sit down and talk to him. But if that's all you do, sit down and talk calmly, like some naive posters here tell you to do, it's just going to happen alongside his side deeper and deeper into these holes. He has to understand that there's a real possibility you'll walk away from him if he doesn't at least look at logic and reason.
Don't listen to the instantly disown crowd, but don't listen to the painfully naive, just talk and love and nothing crowd either.