I'm all for the portable DVD players! Plus, you can turn the volume down low enough that your son can hear it and it won't bother everyone around him (I'm not sure how the headphones would work out).
My family lives in Texas and I live in Oregon, so I've done A LOT of flying with our daughter, both with my husband and alone. If it weren't for M&M's (sorry, but planes get special "treats" when your traveling alone with a small child) and Baby Einstein videos...I don't know what I would have done.
Also, grab a couple of new toys that don't make too much noise and put them in a bag until you "really need them". A new interesting toy always seemed to intertain my daughter for about ten minutes at most. But every minute counts!
And most of all, give yourself a good pep talk, know that you love your child and sometimes there's just nothing you can do about a tantrum on a crowded plane. You'd be surprised how accomodating most people will be too!
Good luck, and let us know what happens!!!