My DS11 has been afraid of flying since he was 4. The fear is intense and pervasive, and it got worse until its peak last summer when he was 10. Here's my two cents based on our experiences.
If he's just afraid of flying but otherwise is not having excessive fears or other issues, then deal with the fear of flying as a fear of flying. Others have posted suggestions about how to deal with that.
However, we tried various techniques, and they did not work. In fact, he was also behaving strangely in other situations--not able to sleep, not able to take a shower unless I stood guard outside the door, and in general, having what I think are panic attacks (though he does not use that word). So flying was just one of many situations in which he was having anxiety and fears.
If your son has other issues with anxiety and unreasonable fears, I would recommend getting a referral (probably from your pediatrician) for a therapist. We initially went to one because of DS's fear of flying which I then realized also included an inability to sleep, fear of being the victim of crime (while in stores, while at home, etc.), and really lots of anxiety in lots of situations. The therapy has been wonderful for him, as in life changing wonderful.
He was diagnosed with general anxiety disorder with elements of OCD and post traumatic stress syndrome (we were inside a Target when five women were murdered in another store just across the parking lot and our subdivision is very close to the scene--lots of trauma there).
He's been in therapy since July, and he's learned lots of techniques that can help him get through the stressful times. Hawaiian music (he pretends he's at the Polynesian) has helped him tremendously, and the therapist really works with him to have other options he can use when he needs to.
So he does not like to fly (neither do I!), but he can do it. He's learned various types of techniques to calm himself down and ways to handle the fears. Also one thing--his psychologist told me that avoiding the fears makes them worse, so just avoiding the problem by driving is not going to help him in the long run. I'm not advising you to force him to do anything, but that's what we were told.
Also, our psychologist referred us to a psychiatrist who recommended meds. We chose not to go that route and found that we made the right choice for us anyway. I just want to stress that medication is not always needed.
One more thing--his therapist said that sometimes, anxiety misdiagnosed as ADHD and vice versa because the symptoms are similar.
Best of luck to you.