Hi Gareth
We've done this to New Zealand and the good news is that it doesn't have to be [too] hard work! The key for us was planning and preparation (of ourselves, of my step-son (then 8) and of the airline).
We flew Air New Zealand (being a loyal Kiwi) my partner is British! but we rang them in advance of booking to discuss what they could do and how we could make the process as stress free as possible. Key things for us to manage were pre-boarding (getting him on the plane and settled ahead of everyone else), thinking about the seat - we gave him a window as it was quieter and there weren't people pushing past. Obviously, prep was vital - what would happen at the airport, on the plane, how many sleeps on the plane, stopovers).
We chose to break the trip up into two legs but were lucky as we have family in LA. That did make it more manageable and by the end I'd say Jack's a veteran flyer - he has since flown to the US again without any problems.
Make sure that whatever airline you choose they are on side with what you need to do. Obviously, what we did won't be the same as you need as all ASD children are different. This brings in the point about making sure you have all the stuff your child needs for comfort and calming. COnsider also sleep issues and medication on the plane (we got a prescription for phenergan). We even took sandwiches as we knew Jack wouldn't eat the airline food :-) And on the subject of which, think too about toileting... 'I need to go now' takes on a whole new meaning when announced in the middle of the dinner service ;-)
Prior to boarding make yourself known at the gate, ask to talk to the in flight manager and ask them to make the crew in your area aware of your childs condition and needs (and your needs to support this). Explain to them what might or might not happen and get them on side... we had some amazing in flight crew and that made it easier for us. We used a wheelchair after the long haul, Jack's mobility was fine but this hasd two benefits, one it reduced his stress and the other was we asked for priority clearance through immigration etc.
Good luck with the trip, sounds like you're getting on to this early, which is great. Do feel free to contact me directly if you want to chat!
Best
Andy