To establish where I’m coming from: I used to work at EPCOT, and did a fair bit of wandering about the parks. However, that was in 96, so things might have changed (I visited three years ago and everything seemed similar, but they might have changed stuff, so please correct me if I say something outdated).
I’d go with len’s itinerary, I’d have suggested something along these lines. EPCOT used to be one of the more relaxed parks, we used to go there in the afternoon and just wander around (and stay for IllumiNations, which is the best fireworks display of the parks IMHO). Just don’t enter the park in the morning and get into line for Spaceship Earth (the big golfball structure), because it sits there looking large and attractive - everybody does so and you’ll be in line forEVER and the ride is rather boring. You can do it in the afternoon with a lot less waiting involved.
To keep a lid on your budget, if you are looking for a quick snack (and you will be, because you will have been walking a lot), the little food carts that serve fruit used to be among the cheapest options. Also, the seafood restaurant behind the Haunted Mansion (forgot the name) used to offer some kind of chowder-in-a-bread-bowl that was reasonable and quite tasty.
In terms of the EPCOT restaurants I’d recommend the Norwegian buffet over the Italian restaurant, which was rather high priced for glorified Olive Garden fare. However, the German buffet also used to suck in 96, and when I saw it three years ago it was really spruced up, so they might have changed other restaurants for the better as well. Usually, restaurants were a lot less crowded (and cheaper) during lunch (still, reservations are a must).
As for the rides, whichever you choose I second the use of Fast Pass for as often as you can get it, it’s a life-saver. Also, while Pirates of the Caribbean might be a classic, it probably has one of the worst lines in the park and you’ll spend a long time trapped in a damp dungeon tunnel in a line that does not seem to move for an eternity. I would think this probably has not improved with the success of the movies…
I personally love to spend time at the Studios, but I just love the Tower of Terror. And the Aerosmith Coaster - if you’re late enough and everybody has left to watch fireworks and the WhatsitsnameSpectacular, you can just exit the one ride and enter the other! But if your son is ambivalent about rides, those might be a bit too scary for him - I’d go with the Mountains in the Magic Kingdom, that should be enough.
Another fun thing with kids is to spot the Hidden Mickeys in the parks.
Also, if the kids get cranky and overtired, try to rid yourself of the thought “We spent a fortune and we still have to go here and there and do this and that and and and…” - it’s easy to get swept up in the feeling you’re missing out on something. Take a deep breath and realize you’re not going to see everything anyway, and then just buy the kids some ice cream and go sit by the fountain at EPCOT, or watch the band / crazy robot trash can in Tomorrowland, or watch people milling by on Hollywood Boulevard. You can always go back some other time and do the stuff you missed.