Lots of good suggestions. We travelled when DS was 5 months to NZ - it was 3 years ago though. From what I recall the way out was v easy, lots of sleeping we got a bassinette seat so he slept most of the time. I bf on take off and landing and also took a bottle of cooled boiled water for use in case he wouldn't bf. If I recall correctly we took 15 nappies (and only used about half as he slept for so long) and had 3 sleepsuits and vests for DS and clean tops for me and DH in the cabin bag. Typically he was sick on my trousers which i hadnt bought spares of.......! If you are on your own you won't want to be carrying a big cabin bag as well as baby.
Definitly take a pashmina, shawl, bf cover as I felt quite exposed on the plane. You are v close to the person next to you - mind you prob not in business class!
I travelled with DH so this is some advice from a friend who went to Aus at the same time. - for you, wear jogging bottoms or something easy to go to the loo in as you may be doing his one handed/ with the baby in a sling. Also take some muesli bars or easy snack as eating might not be easy. The cabin crew were fab for my friend taking her son at meal times and when she needed the loo. When she was in the airport she didn't have that hence the suggestion re jogging bottoms. But she travelled with a sling only - we had the buggy which made things easier in the airport.
I was going to delay weaning until when we were back but it didnt work out that way, Our HV recommended we start weaning at 5 months and although we didn't take her advice immediately it was clear a week into the 3 week holiday she had a point. It was fairly easy though, fresh fruit and veg, we used a few pots as well. For that amount of time I don't think it was so much of an issue.
The way back was less relaxing as he was more awake, wriggly. It's amazing the difference 3 weeks makes at that age. We didn't get a bassinette for part of the journey (8 hours) which I was v upset about but then there was turbulence for almost te entire journey so no one was allowed to use them! That was a hard part of the journey but again he slept n my arms for most of it. My arms were aching as DH was sitting behind me so we couldn't switch easily.
Good luck, I worried about it for months before we went but actually the experience was much easier than I imagined.