Are you flying Qantas? I flew down two years ago-- LA to Sydney-- and it IS a long trip. Qantas, however was great for keeping you occupied. Every seat has its own screen with a selection of movies and even tv shows to watch at your leisure. There are even some video games that you can play, and I noticed a lot of kids enjoying those.
<a target=_blank class=ftalternatingbarlinklarge href="http://www.qantas.com.au/flightinfo/do/dyn/entertainment/international/index">Qantas In-Flight Entertainment</a>
I also suggest a coloring book and/or crayons and paper for drawing (my personal preference), a camera for taking fun pictures as he gets settled in, so later he can remember it all from his own perspective; maybe some travel games, a walkman, madlibs... I hope some of these things are helpful! If he likes to read, get him a book about Oz so he can read about some of the places/things he is going to get to see!
And, in all seriousness, if he has a hard time sleeping in weird places or gets extremely antsy, I'd reccomend having some children's benadryl on hand. Because it really IS a long flight.
Good luck! This is going to be amazing for you guys!