We just returned from WDW with our 2 yo - flying from San Diego! I also fly often to Cincinnati (another 4 1/2 hr flight) This is what I packed: lots of stickers & paper, (after he puts the stickers on, he 'circles' each one with a crayon) crayons, coloring book, books w/flaps inside, those thin square books (they aren't heavy), and animal flash cards. For this trip I bought a tape player ($10 Target) he listened to the headphones for an hour! It was great! He also plays with the buttons on the arm rest, although sometimes annoying if it is for the light.
Call ahead and request a toddler meal, on Delta it was cheese sandwich, banana & cookies. Snacks I bring, Ritz bits, goldfish, teddy grahms.
If at worst just let him walk up & down the aisle a bit, it just may be difficult to get him back in the seat, but I have done this as a last resort before. If you are changing planes, let him run around the airport gate area, climbing on seats, whatever.
Your trip will be fine. Have a great time!