M.M.
Let him run in the airports when you can to burn off some energy.
When we flew with DS at that age, we used everything. Things around the house that you wouldn't think of normally: A band-aid kept my DS busy for 20 minutes alone because it was sticky and "new" to him! A calculator - lots of buttons to push. Or an old cell phone. Chip clips and let him "clip" something.
Go to the dollar store - or the dollar section at Target (on the end caps) and load up on stupid little stuff. I would advise new toys, rather than what he knows, since those aren't as fun. Bring books he likes. And I always keep a couple of episdoes of Mickey Mouse Clubhouse ( a favorite) and now the movie Cars on my video iPod as a backup distraction. (I don't like hauling the DVD player around).