Possessed, huh? That could be totally feasible. And, well, it could also be a good sign that it won't happen again if he gets... 'unpossessed'.
I think the best thing would to just be there for him. Make sure he's not a danger to himself or anyone else. Keep it in his mind that he has friends who care about him.
And, um... I heard weird stuff was happening today around the place, so it could just be a freak thing. Maybe he'll be okay tomorrow.