Who are you flying with? Some companies, like Singapore airlines, allow for Inflight convertible beds, a inflatable cube that allows a toddler to lie flat. Look here https://www.singaporeair.com/en_UK/gb/travel-info/special-assistance/travelling-with-children/ under " seat".
Those didn't exist when my kids were that young, and I would put a trolley and coats on tops to make a make-shift seat extender.
Also make sure to pack a change of clothes for you as well, as toddlers move in unpredictable ways and a glass or whole tray might be knocked on your lap.
In case you are travelling with a lot of snacks, make sure to leave behind all the FRESH food, such as fruit, cucumbers, uneaten sandwiches with ham, and so on. Australia has very strong laws to protect agriculture, so anything fresh you have, leave on the plane to avoid crazy fines. Have pens with you to fill the incoming card. When I had the three kids with me, I always found it easier to have all the passport numbers on a little card with the flight number , instead of having all the passports next to me with the risk a juice getting on them or having to stand up, take bag down, put away.
If you are flying through Hong Kong, there is a second security check at the airplane doors and all liquids will be taken. Not formula of course, so have the water in baby bottles to which you can add formula later during the flight.
If you manage to wean him off the breast and he gets used to a comforter of any kind, buy spares of that one, be it a soft toy, a blanket.