On the other hand, bottleofbeer, I have no idea how I'd eact if I woke up in a hospital bed, in green pyjamas with an empty drip attached to me, on an empty ward, with no recollection of how I got therre, to discover that I have apparently been there for two weeks, to find a lost girl, lots of weird things happening, and all that.
He could be in shock in which case he wouldn't be conducting normal conversation but his mind would be blank (that's how mine goes, anyway!) while it desperately tries to process what is happening.
The point at which zombies appear is the dangerous one from the point of view of the trick. At some point you're bound to think "this is just like being in a film" and it's not a big step from there to "this can't be true, it must be some sort of trick". However, if they can keep him busy enough with not much downtime for reflection then it could be kept going for longer.
This is assuming that Derren didn't implant some reinforcing suggestions when he put the guy bakc to sleep on the bus, along the lines of "everything that happens to you from now on will be true and you will believe it completely", but that is pretty hard to do.
(Yes, I have ms, btw, and I would love db to sort out my malfunctioning immune system!)