Is there some way to slip him a little mickey of some sort? Just kidding. Maybe a full tummy before he goes. I would also take something he is very familiar with (i.e. his cat bed) and let him have that there too.

Gosh, I hope the vet knows how darn unpredictable cats can be.....and how their pursonalities are so subject to the unknown!

Good luck. He seems like the ideal kitty for someone!