A.C.
I can suggest/endorse the Mucinex. My three year old is on her second set of tubes...but will still get ear infections. If I start a 3-4 day course right when the congestion starts, I can prevent ear infections I would say more than 80% of the time. It keeps everything moving and we can ride out the infection without antibiotics--I'll take the Mucinex any day over the alternatives.
For your purposes, I would start 2 days before flying. Make sure he gets the appropriate dose/schedule and is drinking plenty of clear fluids...avoiding as much dairy as possible.
Poor guy. I had the opposite problem...I could tell when one was coming before the doctor could really see anything. We'd end up going to the doctor twice in once week until my Pediatrician figured out I knew what I was talking about.