If it's his first flight, looking out the window will be a HUGE help keeping him still, so make sure he gets a window seat. If neither of your seats have windows, see if the person beside you will switch.
During take-off or landing, give him something to eat, drink, chew, or suck on. The act of swallowing tends to help with the pain of his ears popping when the cabin is pressurized and the pressure changes.
One he gets over that, coloring is always good. If you get him brand new crayons and coloring books, the treat of having something brand new will help too. If he likes stuffed animals, bring one or two. Maybe bring a book or two and read to him, if he gets bored. Any small craft-like things that don't make too much of a mess are good. Whatever he does at home quietly, can usually be modified with a couple new rules for a plane ride. If you own a laptop maybe bring one of his favorite movies. Or if you've got an ipod or something that can play videos, that too. Along the same lines as that, with a computer/mp3 player maybe get some of his favorite music to bring along, or get a walkman for him. With all of these things, keep in mind the people sitting near you aren't going to want to listen to them, so make sure he has over-ear headphones, and that he knows better than to turn the volume up.
Snacks are a must have, but airlines don't let you bring liquids or fruits through security anymore, so watch what you pack. Crackers, and pretzels are what I usually bring for kids, but you know your son better than we do. Just bring whatever he likes that's non-liquid and not fruit.
If he gets tired, encourage him to take a nap.