OP, it seems that your son has agreed to go with medication so I'm happy that you will get your trip. I thought I would still post anyway because I too have an extreme fear of flying. Do not be surprised or upset if he changes his mind and decides he can't fly after all. I loved to fly growing up but one day I began to develop anxiety about flying and all kinds of other things. I have gone to Disney every single year since I was two and I am almost 33. This is the first year that I have absolutely refused to step foot in the airport or plane. We are taking a large family trip and I told everyone I would drive from Virginia to Florida alone while everyone else flies because I don't want to inconvenience anyone. They of course said no and my dad agreed to drive the 14 hours with me (even though he isn't thrilled). The purpose of my post is to say that no matter what you tell your son, it won't matter and can sometimes make things worse. People who are afraid to fly know all of the stuff like, flying is the safest form of travel, look how many flights there are per day and how few problems, blah, blah, blah. None of this logic helps a person who has fear/anxiety about flying. After spending just about every flight for the last 8 or so years in tears, it just isn't worth it anymore. Embrace the fear of your loved ones and realize that if they could change it, they would. It means far more to someone if you agree to suck it up and drive the long distance rather than making them feel like the only way to have a trip is to fly. People who have never experienced extreme anxiety or fear related to flying will never understand what it's like.
