I have an autistic son who has challenging behaviour, can you recommend something to calm for the flight?

Depends on age: Depending on the age of your son and the level of functioning the response may be completely different. Younger children may do well with a hug and some deep pressure to relax them and older children may do better if left alone or moved to another area that is safe to let them settle down on their own. A behavioral therapist can help you develop a response that works the best for you and your son.