It's practically a genre convention that in a speculative fiction story, if someone says something weird or unbelievable is true, they turn out to be right.
In the old days, the surprise twist ending "the aliens are actually real!" or "he really can do magic!" was sufficient to make a story. These days, that's not a surprise; it's what most sophisticated sf readers will expect.
So "He's not crazy, he really can see ghosts" is an insufficient premise for a story these days. You gotta do more than that.
Of course, the obvious reverse twist ("He really is crazy after all!") is hardly better. Complexify.
In general, I feel that a story should be about more than just messing with reader expectations. Which is a lot of why surprise twist endings in general are just not my cup of tea these days.