do not scare yourself too much.
your insureance will make deal with him.
if he is paid by your insurance, no matter what, and he still try to sue you for the rest, his case is very very weak, can not win.
take my words, and tell him this. a cheap lawyer can handle this. remenber, he will have a very very weak case, if he really so stupid to do so.
there are many points to defend you, just pick one, he is the signer ,too. so he has to be responsible for anything happen to this travel. and many others, such as to challenge the medical record, or he is the one inspired to go this travel, and so on so forth...
up to the sue, it is the technic that makes the difference.
to many to be mentioned, let us know what is going on, and i tell you what to do.