Bring your own food.

Buy bulkhead seats [more leg/play room, can't kick seat in front of him] and give your child the window seat. Choose a flight that will not be full of commuters. Point to landmarks out the window.

Arrive early so you are not stressed by check-in. Even earlier if this is an international flight. Have doctor notes for anything resembling medication. Check with embassy/customs before the flight, if this is international. Find out what you can't bring to your destination country.

Do NOT preboard your child. Have one member of your party preboard and get all your carry-on items on the plane, but keep your child off until the last minute. The flight is long enough, why make it longer?

Bring lots of new toys, etc. Wrap them like presents. Give them out one every 60 minutes or so, hopefully tied to "you have been very good on this looooooong ride".

Find out if there is a movie and what it might be. Or bring your own DVD player.

Is he verbal? Bring games, books, etc. Some kids are map-savant, bring maps.

I have read good stories about giving a sleeping medication [know IN ADVANCE that it will cause sleep instead of hyper] and the child sleeps for the first several hours or more.