Budget around $15 per meal, per person. That will get you a cheeseburger, hotdog, or chicken strips with drink and fries. It's not all burgers and fries though. There are also a few places that serve better food for around the same price - there somewhere in Orleans Quarter that has decent sandwiches, salads, and soup bowls served cafeteria sytle for around $15 +/- a few $. The Main Street circle has a place with decent pastries and coffees. Blue Bayou (sitdown restaurant inside the Pirates ride) is expensive and you may need reservations. Budget around $4 per drink (like bottled water) and $6 per snack (churros/pretzels/ice cream/etc). You are looking at around $100/day in food if you eat in the park fortwo mealswith a few snacks and drinks thrown in-between. Keep in mind you can also leave the park to eat and rest then return with a hand stamp. Downtown Disney (just outside the park entrances) has a few places to eat and shop too.
You'll probably also need to budget for souvenirs like Mickey Ears, shirts, stuffed toys, etc Don't be a tool and buy those early in the day and carry around a 4 foot stuffed Goofy all day. Unless you take them back to your hotel.
Keep in mind your son might not meet the height requirements for some of the "adult" rides.
Plan around any parades or fireworks shows you want to see, you'll probably need to be in place 30+ minutes to get a decent view spot (probably earlier). The good viewing spots tend to fill up early along the routes.
Let your kid run around on Sawyer Island. He'll probably have just as much fun there as on any ride.
Watch out for water rides at night in December. Do them earlier when you can dry off.
Look up the times for the Jedi Academy, where your son gets to wear a Jedi robe, wield a light saber, and go on stage to battle Darth Vader. They do it a few times a day but it's mostly early (like no shows after 4pm or something).
Fast pass can save time to be sure, but Disney does a very good job of making the lines interesting. I probably like all the visuals and details in the Star Tours and Indiana Jones lines better than the actual rides.
Don't be a dick with a cell phone or camera flash in the haunted house elevator. In December, it's still Nightmare Before Christmas themed too, which is cool.
Ride the Disney Train around the park. Took me 30 years before I did and it's cool! I'm sure your son will like the Dinosaurs.
You'll learn to hate strollers and to really hate powered scooters all the overweight people ride there.