My friends' two sons have autism and ADHD but she has taken them on several long haul flights. She packs those travel packs (I think made by Crayola) that you can get in Woolworths or WH Smiths. She also takes Gameboys, top trumps or Pokemon cards. I know she usually tries to get the bulkhead seats and because her two are special needs she gets boarded first and is taken to a separate check in to avoid the boring queues which tend to be a trigger point for the boys playing up.
By the way phenergen gets my daughter wired and certainly doesn't relax her. Have you thought about a herbal remedy like Rescue Remedies or Kalms? Lavender oil also works as a relaxant - I rub it on my little girls wrists at night to help her get to sleep.