Pembquist's suggestion is a potential way of dealing with it. I've got some friends with a few funny ideas too, thankfully not having reached the level of an 'obsession', but trying to steer them away into other subjects/activities is a possibility.
Unfortunately, sometimes it's really difficult to do and doesn't always work.
One question I'd ask myself if I were you is, are you willing to look past his funny ideas to still be friends with him? I don't mean it as, 'You've got to be more openminded and accepting of other ideas!' but just, can you live with him being deluded and still be friends? Possibly move past trying to set him straight and pull him back to reality?
I know for myself there's a certain level of WTF that will just make me go, 'Nope, you're not worth it.' I guess it's up to you where to draw that line with this guy.
Sucks when that kind of thing happens. Best of luck with it.
Wiley.