So sorry how things are going for your husband. I am gradually speaking to more & more people about RT & ORN and not one person has been told about this side effect. And I am getting to feel quite angry about it. It is an enormous penalty clause and had we been fully informed not only of the existence of ORN but its implications as well, we may have been able to go for other options - for example having all teeth out prior to RT (which seems to be fairly standard at some hospitals) or have the oxygen treatment alongside or immediately after RT. But we were not given that option.
I have read on one HBOT providers site that for very nervous people they are introduced slowly to the chamber and can give 'calming' medication if necessary (I think this was one of the Spire Hospital sites). It does seem to be the only thing that can halt/stop the progress of ORN. Even at an MS therapy centre they may well be able to let your husband get used to the chamber at a quiet time, just to sit in there with the door open to get used to it may help. It is well worth getting more information on how the claustrophobia can be helped. There will be a Ms therapy centre in Surrey http://www.msntc.org.uk/centresearch.php
it might be worth contacting them and I'm sure they will have experienced clients with claustrophobia before.
Thinking of you both, x