Someone told me a tale from their pentecostal days. They were all the 'hand waving bible fearing' types that 'drove the devil' out of the worshipers with prayer.
One woman was cured from demonic possession, with similar symptoms, almost every week.
Ever think it might be self hypnosis with socially conditioned expectations?
Again, social conformity, and expectations. Ever wonder why it only seems to happen in prayer sessions??? People get worked up... and it's a communal experience.
However, does the one that started biting himself and talking in many voices belong to a church that has similar traditions?? Do they have people who 'speak in tongues? Can you relay what his Christian denomination background is?