Can't begin to tell you how surpirsed I was to see someone jump to the conclussion that your child is formula fed?!! I had three children ALL with severe reflux & breastfed each of them until well into the second year. For the record formula makes NO difference to a reflux baby vomiting - infact it makes it smell worse! Whatever you do, DO NOT restrict your childs fluid intake before or during the flight (if anything increase it) as flying is very dehydrating and refluxers are more prone to dehydration than non-refluxers anyway.
Now back to the topic. Ours were all extensively travelled & unfortuntately just like at home, there is little you can do on an airplane to prevent the vomiting. Just be prepared with plenty of quick drying muslin cloths to mop up the spills & wet wipes. I always sat mine on my lap when the seat belt sign was off & they weren't asleep. That said they all tended to sleep much better (including the two that vomited from feed to feed for 12 months!!) whenever we flew. A rolled up towel or an airline pillow under the bassinette insert will help with elevation of a flat bottomed sleeper.
Good luck - it won't be as bad as you are anticipating!
