We have been flying with our son to WDW twice per year since he was 6 weeks (not as long as yours since we are flying from Chicago - about 3 hrs on the plane). One thing we have done from the beginning is that other than a couple trips to the restroom, our rule is that he MUST be seated and belted in. This is both a sanity and a safety rule. I used to travel extensively for work and way too many times I saw "flying" children in plane cabins during sudden turbulence. This was mostly lap-held babies, but I have also seen small children really thrown around! Having him IN his seat, does make it easier to cope.
If there is a movie and it is at all reasonable for a kid, we get him headsets. (Of course, if you have the portable DVD player others have mentioned, that is the same idea - we just got our first home DVD player a few months ago and haven't even considered a portable one!)
Some of the things we bring...
Coloring books, blank paper, crayons
Pipe cleaners - nice and light, but they can make shapes out of them for ages.
Playdoh - usually just one color & maybe a couple cookie cutters
A few small, paperback picture books. (Cheerios recently had some great ones in the boxes!)
Sticker books, Activity books, or just stickers and more paper.
Basically, anything small and light that will hopefully occupy him for more than 5 minutes each!
Good luck
Pamela