That does show great distress - did you not warn your son someone was coming to visit?
I can only suggest buying a small pop up tent and when someone comes, put the tent on your son's bed so he can sit in it with anything he finds comforting, and give strict instructions that no one under any pretext is to go into the room - barricade the door from the outside if you have anyone stupid coming.
Ensure the visit is short, and tell your son that there will be a special signal which he can tell you for when it is safe to come out - suggest secret knocks or a whistle. That should both safeguard the visitor and stop your son being classified as violent and dangerous.
Don't ever forget the secret signal. Practice the routine for five minutes, first with you standing outside the door to speak to him, then gradually increase the time until you can 'do it for real' - maybe have something delivered?
There are numerous coping strategies - some incredibly simple and some quite fiendishly clever to help frightened children - I have only read others reporting of them, but maybe there is a forum similar to this one?