ok, the first thing to rememebr is that it will be tiring! but it is doable.
we have flown longhaul several times with our 2, and it works ok.
you have a younger ds too, don't you? will he be coming too? book bulkhead seats, and a skycot/toddler seat as a must. having somewhere safe to put the baby/toddler is a godsend.
ipad good. I ahve found sticker books to be good as well. I also found (ELC I htink) a HUngry Caterpillar soft fabric book, which came with fabric pens to colour in the book - good thing about this is that once you get to holiday destination, it is washable, so you can rinse/repeat all holiday! if sticker books no good, then the magnetic sets can be good, or fuzzy felt.
pushchair: is this 'just' for the airport, or will you need it at destination too? if jsut the airport waiting, is a sling an option? much easier to keep with you and have immediately on arrival. a soft structured carrier (eg Ergo or similar design) saved our bacon more than once (and dd1 was not small - last time we used it - well, dh did! - dd1 was about 4, and weighed around 17kg. but easier to carry her on his back, strapped in, than to have to try to soldier on just picking her up and balancing on hip)
if you do need a pushchair, then it can be possible to get it on board - dh managed a couple fo times ot get out Phil and Teds stowed on board, but it doesn't always work. alternatively, you could notify the airline of disability, and ask for ssistance getting form the aircraft to the terminal/baggage hall. a wheelchair can be aranged, but it will likely be an adult sized on, with a lapbelt only. so depends on whether your ds would be ok sitting still-ish on a big seat with not much restraint!
food: taking your own is the best bet, I have found. we have to anyway, as dd1 is gf/cf, and the airlines cannot cater. security restrictions do state not liquid foods, as it counts as part of the liquid restrictions, but we have always managed. we have a letter from our gp saying we need to take food due to autism and dietary restrictions, and this, coupled with offering to taste etc, has meant we have pretty much taken whatever we want. fish pie in a thermos, curry/rice in a thermos, anythign really. obviously, if sandwiches/easy snacks are possible for you, this woudl be an easier route to take. we also had trouble with drinks, as dd1 cannot have apple/orange juice and can only drink from one type of sports bottle. so we had to bring our own, and in quantities greater than 100ml. again, the letter form the gp helped with this, although you do still get funny looks, and it takes extra time at security etc.
one thing that we neglected to do on our first flight with an aware dd1 was to practice the security screening, including handing over her comforter/whatever object she was carrying. that caused a bit of trouble...