Are you up for a bit of bribery and corruption? Find out from the airline what they offer, if they have the screens in the back of the seats so that he can choose his own film to watch, most will have some kind of pack with crayons and paper, activities etc. Buy something new to open once on the plane, have a stash of sweets ready for take off and landing to help with ears. You may find he surprises you and copes fine when he sees everyone else sitting down he may want to do the same. Don't try to keep him in his seat the whole time, take him for walks up and down the plane to see the flight attendants, see where they work, sit for take off etc, most of them are pretty good with small children. And don't forget to try to stay calm yourself!