I had same concern with DD8 last year. ADHD and first time flying. I gave her some gravol (first time, child dose) and some cough syrup (recommended by dr. since she was just finishing up with an ear infection) and antibiotics (for an ear infection). WELL, DD is not medicated for the ADHD and has had very little meds (for colds etc) throughout her life. The combination of these meds (plus having to get up at 3am for flight) knocked her out and she slept for the entire flight. I felt so upset with myself since she missed most of her first ever flight. I AM not suggesting you do this med combination, but I would check with the doctor and see what he/she recommends. Next flight I am going to skip the gravol, since our flight I have read that it makes a lot of kids VERY sleepy.
Maybe you could also consider about reading your child stories about kids on planes and possibly visit the airport before the flight.
Suzy V.