Either he's seen a movie or game when you weren't aware, and it freaked him out. Maybe a movie involving killing or kids, and he felt scared.

I'd take him to a pediatrician because this is causing you and him distress, and isn't going away yet. If you could find the source that'd be great.

Have you tried playing along with him? Asking him things like "why" or "what do they/him/her tell you?" and basic questions to get him to give clues about the source.

About what he said about being normal, what is normal and what part is unwell? Either way, if you want to get inside someone's head, try to play along. Let him know you're there to support AND you believe him. Doesn't matter if he says an octopus will come through the lamp and eat you. The more you support, the more you (and doctors) can figure out, and hopefully be with him step-for-step.

How is he around other kids? Is he also like this or does he snap to being all fine and bouncing around?