I have just returned from Australia with my 20 month old DD. I also took her there at 4 months.
In addition to the good advice you have already been given, take some favourite toys and also have a stash of new toys / books to keep your DS occupied.
Each time our flight left the UK at about 9.30pm. DD fell asleep shortly after take-off and had her usual long night-time sleep for most of the first part of the journey (first time to Singapore, this time to Hong Kong). She was awake for more of the second flight, but I found this way round easier to deal with. Get as much sleep as you can when your DS sleeps.
Don't try to adjust your DS's sleeping pattern on the flight to fit the new time zone - just go with the flow and accept you will have a few middle of the night parties when you arrive in Oz. I find it easier to get over jet-lag there by getting out in the sunshine (I am really suffering with jet-lag at the moment because it is too cold and dark here).
To help your DS cope with the change in air pressure (ear ache) make sure he is feeding during take-off and landing or sucking on a dummy. If he does start to cry due to his ears hurting, don't worry, crying also helps to equalise the ears.
Finally, whatever happens, remember that the flight is only 24 hours of your life, you won't know the other passengers and you will never meet them again. So don't worry too much about your DS crying at times.