Good luck with the flight... I've always thought that from crawling-2 is the worst age to take a child on a plane - but you've got a seat for him so you avoid most of the problems I've always faced with that age (fly frequently with four under-7s). Very good decision! The other thing that works to your advantage is that it's long haul, so most of the time (one hopes) the seatbelt sign will be off and he'll have a bit of freedom.
My girls were usually amused with books and a quiet singsong (and any toy that the airline might - or might not - provide), but my boy was not quite so placid. My best tip from my experiences travelling with him is this: when nothing else is working, and he's about to annoy all the people around you, suggest he takes his shoes off. They often like that at that age, anyway. Never mind if he doesn't play ball, whip his shoe off and turn his sock into a glove puppet, complete with munching noises as it tries to eat his toes, tummy, arm, nose... Never played this game anywhere else with DS, but it worked like a charm whenever I needed it too.
Just pray you don't get locked into a holding pattern at the end... but that's cruel of me to suggest the idea!
Other tips:
Check whether they'll provide food for him - they usually don't for infants (except useless little puree jars), but if he has his own seat it may be different. Hopefully it is.
I had another one but I've forgotten it.
Oh, and spare a thought for those of us who don't get the extra seat, and have wriggly toddlers at our knees, with their hands in our food and then on our clothes and then half the food on the floor and no room to play because the trolley comes past every time they get into the aisle... My youngest is 7 months and it's all about to begin again with her - not looking forward to it!
Anyway, to end on a positive note, there are lots of distractions on a plane and often lots of people willing to help out, play with DC etc - plus any time that they sleep feels like a bonus - bliss!
HTH and hope I haven't gone on too much...