I think the most important thing is preparation: Flying for the first time is exciting, especially for small children.
Wiggling on your seat as a kid is usually a sign of being excited, restless or anxious. If you can, go with him to an airport, have a walk around, tell him what is going to happen when and where. What happens to your suitcase, what is a flight attendant. Show him videos or photos of the inside of the plane, that there is a little kitchen, there are bathrooms, etc. What are the rules on board: stay seated during taxi, take off and landing, and when the 'fasten your seatbelt' sign is switched on. But that (if needed) he can go to the bathroom / walk around in between these periods. Then he knows he needs to stay seated for at least 10-20 minutes (depending on the airport / gate/ runway) at the beginning and end. He's probably too young to understand the concept of time, but if you for example bring a tablet with short videos of about 10 minutes, then you can tell him, after one or two videos the signs will be switched off.
When children know what to expect, they are usually less restless / anxious. Be careful not to protect your fear on him. When you tell him 'this is going to be scary', he is more likely to be scared. I would explain about the pressure on his ears (maybe he knows this feeling from riding through tunnels, or maybe when swimming?), but would be careful to explain things like turbulence or air pockets.