Last year i took my 2 children to australia from the UK, my son was 6 and is autistic, my daughter is NT. This is about a 23hour flight and i was honestly dreading it. We were going to a family wedding or i wouldn't have dreamed of taking him on such a long flight for his first time travelling.
Like the above poster i made sure the airline was aware of my son's autism, and we were allowed to board first and leave last on the airplane. The airport also provided a special information pack for autistic children that explained all the different procedures. We also arranged for a wheelchair to be available at singapore when we changed planes (he is perfectly able to walk but we felt he would be better sitting and pushed as he would be very tired and would otherwise have to be carried). I filled my bag with snacks, drinks and he had his DS with him and his ear protectors and special blanket. We took pullups with us and spare clothes in case of accidents. We took his favorite soft toy with us too.
He was enthralled with the onboard entertainment screens in front of him. In fact he coped with the travelling better than the rest of the family Slept well, had no meltdowns (which is more than can be said for me, i was drained by the end of it). I actually think the noise of the plane, which i thought would irritate him, relaxed him...
Anyway, plan ahead, research any and all help available, and good luck