OK here are a few tips from a former flight attendant.
Don't put him in the window seat! Best bet the aisle, what he can't see is less likely to scare. If he likes it after take-off you can always switch seats.
I would use dramamine if he has had problems with car sickness. Carsickness doesn't mean he'll get air sick but he may have a problem. It makes them a little drowsy too so he may be less bothered by the first experience. Benadryl won't help the motion sickness. Also keep him cool. I know from experience if you over heat you are more likely to get airsick.
Ask if he can visit with the pilots before the flight. If he gets a chance to speak to them and see the cockpit after you board and before the doors are closed it may fascinate him enough that he forgets about his fears. Most pilots are happy to visit with kids and give them a look around, especially when it's a first flight.
Ask the pilot or a flight attendant about getting a set of wings and a first flight certificate. Most airlines have at least the wings. Again he may think this is so cool he'll forget about his fears.
Try not to make a big deal about the whole process and overwhelm him with things that can happen on a flight like turbulence. This will only magnify his concerns. Should they happen during the flight then you can deal with it then.
Pay attention to the FA's when they do their safety demo so you know where the exits are and what to do in an emergency, xomething you really want to know when traveling with your kids anyway. If he freaks out tell him it's ok, and you know just what to stay safe during a flight because the FA's have told you. Don't talk about an emergency only in general terms of knowledge. If he feels you are knowledgeable and unconcerned he is going to calm down and be fine.
Give him something to do to keep him occupied during taxi and take-off, and don't ask him to look out the window at the other planes. Just act like it's not even there and tell your DD to look out if she wants but refrain from saying anything this time around. Next time if he handled it fine then by all means have at it.
I have yet to see a child who didn't have fun flying once they had been on a plane. He is simply wary of the unknown. If it makes you feel any better my sister's son was around the same age, and had flown a number of times, when their flight got caught in a thunderstorm on landing. It was of course bumpy, and my nephew screamed at the top of his lungs, "We're going to crash!"

. She was mortified

. He is a great flyer now. If he doesn't react like that you are home free

.
Have a good flight. I hope he loves it

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