Does he/did he ever take anti malarial medication eg Mefloquine (Lariam TM)? Common cause.
I did, and used to get them a lot.
The most common proximal precipitator of them was a sudden adjustment in time of turning in, eg say it was 2 am every morning and then suddenly I started to go to bed at 11pm, it would happen. It is extremely unpleasant.
So I go to bed at the same time now. Also, the room is dark. Like no lights whatsoever, not even a PC monitor light.
There are probably counteractive drugs available from GP.
Maybe try sleeping in a different room (him).
Maybe try watching tv in the room before going to sleep, that way he's distracted and doesnt drift off thinking about the hallucination that will wake him (thus inviting it).
Hope this helps. I didnt get it anything like as bad as you describe, but even what I had was v v v sht. I hated going to bed really.