you should just give him more attention on the flight . Have games ready that can be done on a plane with him. Bring his favorite book. Bring his favorite snacks and candy. Make it so much fun for him to stay put that he won't want to leave his seat.
Find a way to hold on to him like with him wearing a belt and you grabbing it at all times. You can always have it a little lose; so, you can have your hand around it.
There may be a child's version of some kind of anti anxiety medicine or a tranquilizer for the flight.
Make sure you go to the restroom with him just before the flight; so, hopefully, you won't have to get up and go on the airplane. Wear a diaper on him, too. Avoid any drinks at least an hour before the flight. Try to avoid drinking on the plane, too. This may mean you can't have a drink, either (show by example).
I would get him very tired before the flight. Walk around the airport a lot before your flight. You may get lucky and he will sleep on the flight. It may be good to tire him out the night before, too.