I would ask him why he is afraid. It may be that he saw a tv news report about airplanes (ie, 9/11) or even a cartoon that shows airplane trouble. Don't assume, just ask him what it is that is worrying him. It might be something as simple as where do you go potty on the plane, or worry about climbing a flight of stairs to get on the plane. Play up the fun, the stewardess will be able to give him "wings" if you ask. Have him help you shop for and pack snacks and games to play on the plane. You might want to buy prepackaged snacks to help with security issues. Let him pick out his own "carry on" with all of his stuff.
Otherwise, don't talk about possible concerns in front of him (lost luggage, etc). I would play down the whole airplane thing and talk about the end destination thing. Also, maybe contact some of your friends who have kids his age, who have flown. Get the kids together and casually start talking about how excited you are in front of the kids, maybe the other 5 yr old will talk about how great the flight and vacation was to your son. But be careful it is not too contrived or he will be on to you.
I have a 13 yr old son who is a worrier to the max! When he was your son's age, these are things that have worked for us when we have faced "scary things". I hope that they work for you. I will be praying for you too.