I would believe him. Others have had encounters with entities that they seem to explicitly know are Other, and aren't just voices in the head. Mind isn't confined to the brain, even if mostly, in a sober state of mind. With psychedelics... Mind is somewhat released from those limitations.

I've heard external voices as well, except, in my case, it was more like, the unconscious aspect of my mind was translating the intention of the spirits into words that I could understand, according to them. When I am mentally tired, I can't hear those voices very clearly, nor sense their presences very well.