You are already doing the best you can to reassure him. I wish I had someone has caring like you when I travel (I travel a lot for work and often alone, even on leisure flights), so he should understand that you have a bladder problem and I think he can deal with you leaving him a few times for a couple of minutes, you aren't going anywhere and will be back.
Me, I have fear of flying (but I like planes, the technology in them and know how they work), I absolutely hate flying even so I've been all over and fly a lot, in my case my doctor gives me either Xanax, or Lectoplam or Valium, I know they are chemicals but they do help.
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Beauty will save the world -- Fyodor Dostoevsky