I'm sorry that you're going tomorrow. It might be nicer to have a couple of days up your sleeve so you could visit your doctor with him just to have a discussion.
I have heaps of anxiety and some phobias, and I have a fear of flying which comes and goes.
Its been gone for a while now, but I would work myself up so much before a work trip that I couldn't sleep, and I would fixate on ways to cancel the trip / meeting without anybody getting annoyed.
For me, my fear of flying came down more to my social phobia - I don't cope well with having to sit right next to a person. I spent one Brisbane to Melbourne flight sobbing into the window, my entire body shoved into it, it was quite absurd and I don't want to know what the poor people next to me were thinking...
The thing that snapped me out of it was one day I was sitting there freaking out, and I realised that a very dear colleage was, at that very minute, on an operating table having a cancer removed, and that she was then facing a gruelling treatment.
I rationalised with myself that I would much rather be having a lovely plane ride than be on an operating table, and I somehow got over the whole thing.
I still have the crippling social phobia but it doesn't manifest in a fear of flying any more.
Anyway, the point of my long and boring anecdote is that your son might be having quite a complicated anxious or phobic response to flying which he hasn't even figured out for himself, let alone been able to articulate to you.
Its not like he can have a nice calming glass of wine
He is letting his fear of the travel overtake his ability to see enjoyment in the holiday, so I doubt that any amount of trying to get him to see the benefits is going to help.
Instead, I would try to play a bit on his conscience, and tell him that most children would be happy to have such an opportunity, many don't get it, and then help him find some ways of coping with his distress. You might want to try getting him a guided meditation or relaxation track to listen to and focus on, or buy him a game or book that he can focus on. Let him know that its OK to have the scary feelings, and that you want to help him find ways to cope with them.
I might be totally off, but that's what I'd try. I really hope that he can find a way through his fear to go on holiday with you.