speaking as a fellow sufferer with a wonky gallbladder, tell frankus he has my full sympathies. *snuzzles him*

if he has a relapse, won't the cattery people be able to deal with it? do they have his vet's number? they should be able to call you/the vet if something comes up and give him appropriate treatment. (which, of course, does nothing at all to soothe your nerves, or your desire to be with him while he's ill, but might still allow your travel plans to go ahead unconflicted)