Hi Alex, great advice for you on here. I travelled with my autistic nephew last year and by the way he loved it; as others are recommending, maximising the routine nature of events is key - it follows an essentially predictable sequence.
The only unforeseen element we encountered was that we were bussed to a remote gate to board the aircraft. Usually at major airports this won't happen, as an air-bridge connects you directly. Sometimes you get a bus though, and at certain airports this is the norm. The buses can be a bit of a squash. Again, you can research the likelihood of this in advance for your particular airport.
He was fine with the bus, we just stood right by the door facing outwards and enjoyed the sights as the apron passed by. Have a great flight.