A few more tips from a seasoned long-hauler:
think about bringing a car seat which is flight approved - kids are used to being buckeld up in them and don't seem to complain as much as a seat buckle - plus extra height and head support are more comfy - make sure it will fit in the seat, though - any anyway, you'll need it if you're driving at all in the US, as all states, I think, require children in car seats
take a change of clothes not just for your toddler, but for you/DH too, as messes have a way of getting all over ... nothing worse than sitting for hours in an apple-juice soaked tshirt (yes, am speaking from experience!)
bring LOTS of little toy suprises and wrap them in tissue or wrapping paper (unwrapping is half the fun!) - about one per hour is our rule - they can be super cheap things, but best not to have too many parts. We usually go for Mr Men books, crayons/paper, small Playmobil set, little toy cars, tea set and the like.
think of what special snacks might really be a good treat - our daughter, for example, loves having "brown bubbles" which is essentially 2/3 fizzy water and 1/3 Coke - not very healthy, I know, but a rare treat and definitely incentive to behave - also, gummy bears. I would bring lots of snack food and water/juice, since kids meals are not reliable (sometimes not loaded, even when ordered, and other times, full of junk food) and anyway, not always served when needed. Healthy snacks plus treats are a good combo.
Depending on where your son is on toilet training, I would consider packing a couple of pull ups or similar, just in case. Even when our daughter was pretty reliably toilet trained, we had to throw her into a pull-up one bumpy flight where she really tried to wait for a bathroom visit, but we weren't able to leave our seats due to tubulence. We put her in a pull up right there at the seat and even then had to convince her it was ok to have a pee ... it really saved us, though!
Space permitting, bring a full size pillow and a nice fleecy blanket - really helps tuck them in and encourage sleeping - especially on the return overnight flight.
If you have one, or can borrow on, a portable DVD player and some cartoons would be VERY VERY good - look into whether you can plug it into the seat, though, otherwise, battery life is only a few hours. Some airlines, like Continental, have a family channel on the flight entertainment system, which is almost all cartoons, anyway, so look into that on the airline website.
Happy flying!