Have you asked to see the spot on his mailbox? What if, like me, you don't have a mailbox? The mailman comes to the door and pushes it through the slot on the side after getting the outgoing mail we've stuck in from our side.
I'm afraid if he's delusional and has latched onto this website as a focus of his delusion, but can't be made to see that he has a problem, he'll never be helpable. But it may not be a full-on psychiatric problem. Lots of unfalsifiable belief systems that people outside the system don't buy into don't reach the point of interfering with daily life. Functionally, this is like his being born again into one of the evangelical faiths - he's all excited about it, and worried that if he can't convince those he loves of the truth of his belief they're doomed to hellfire or endless transmigration as lower lifeforms or whatever. That can be gotten past.
Being separated from his family and friends by his belief system is a Bad Thing (unless, as sometimes happens, his family and friends needed to be shut out of his life; that's not the way to bet). Talk to him about it as a phemomenon separate from his belief set, making the conversation about you and not about him. "Look, I know you're excited about this and you think it's important, but I'm not seeing it and honestly the more you try to sell me on it the more my knee-jerk reaction is to reject it. Could we talk about something else for awhile, go do this thing we used to do, and forget about it for now? If you turn up something new and feel like you really need to tell me once in awhile, I'll try to give it a fair hearing, but it can't be all we talk about." That way, if things start to go badly wrong, he'll still have you for a lifeline and not be in the position of having lost all his easy options except going into full-on cult mode.