Hi laura,
I have taken both my boys on flights though at that age it was a shorter flight, we also took them to the states when the youngestt was a year older than that.
I would say that you need to think of it in stages and change activities every so often. When you first get on the flight, you can use what is already there to occupy him, pointing things out, looking at the inflight magazine etc.
Then you have the thrill of the the take off - when that is finished you could maybe have a little snack to use up some time. etc
What I would have is his own little back pack full of a couple of favourite small toys and a few new ones - little hot wheels cars can keep them occupied - if you can make a little road on a piece or cardboard as well that will help.
What about a little magnetic drawing board if you dont want to risk crayons?
A couple of books, a few songs - would he understand eye spy but with colours?
If you mix different activities - moving on to the next when boredom begins to set in. Remember you have stewards coming round with meals etc to break up the time. Would he sit and watch a dvd on one of those mini players?
Sometimes there are other kids wanting to play peeka boo through the seat gaps,
And a trip to the toilet to get changed is a mini adventure in itself!
He may even sleep for some of the time!
Good luck and enjoy your holiday.