Does your DS like fairground rides? I always used to view the take off itself like the start of a thrilling ride, anticipating the moment of take off as the thrill bit. If he could focus on it as a pleasurable thing, and remember that he feels fine once airborne. Get him involved in planning what he wants to pack, and choosing books/games/puzzles for the flight.
I think he is probably a bit over excited about going on holiday. Try to keep things calm in the run-up to the departure date, he may be sensing tension or anxiety from you. Make sure there is no rushing to get to the airport. Try to keep him occupied during the wait airside, as sitting stressing won't help. Perhaps talking through all the procedures in advance and going over them all again as you do them at the airport will help. Browsing for comics and magazines in the airside shops, and sweets to help with ear discomfort. If this is a problem you can get special earplugs which help release the pressure in the ears on take off and landing, I use them for every flight and they are very good. If sickness is a problem, make sure he takes appropriate medication. Maybe he is anxious about the destination, again lots of information about it can help allay fears.
My DH gets a bit excited on the day we go away, and I am the one who calms him down! He is in and out of the loo, worrying about finding a carpark space at the airport, whether the flight will be delayed, or the queue to check in will be colossal, all things beyond his control. He tends to calm down a bit once we are airside. I make sure before we leave the house that I show him the passports, tickets, travel docs/insurance, money, and he watches me lock the door, having checked all the windows are locked etc. as I know he worries about these things.