You can try checking out "Unofficial Guide to Walt Disney World" - they come out with a new edition every year and it contains a wealth of information and helpful tips about doing WDW, as well as a section on Universal.
At the WDW parks, be sure to get there at least 30 minutes before the park opens to get in line to enter. Seriously. If you are there for right when they open, you will have the best chance of hitting most of the major attractions and popular rides while not getting stuck in crazy long lines. It gets really crowded later in the day because of people who decide to sleep in, lolly-gag over breakfast, and then show up for 11am. Take advantage of Fast Passes whenever you can. These are basically tickets for certain rides that offer them, that allow you to come back within a certain time frame, and then go into the Fast Pass line instead of the Stand-By line, saving you a ton of time waiting in line. At WDW, they cost nothing extra, and are including in the cost of the park ticket, but at Universal, that might not be the case. When we've gone as a family, I hit the popular kiddie rides with DD first, like Dumbo and Peter Pan, while DH heads over to get Fast Passes for other stuff. We've got it down to where we don't wait in line for anything any longer than 15 minutes - and that included Christmas Day which is one of the most crowded days of the year. "Unofficial Guide" even has different "touring plans" that give you an outline of which rides to hit in which order in order to keep your standing in line to a minimum. Always go in with a strategy.
If your son likes roller coasters, at MK, Space Mountain, Splash Mountain, and Big Thunder Mountain Railroad are musts. Pirates of the Caribbean and Haunted Mansion are fun too. My daughter is 5 and also really likes Buzz Lightyear's Space Ranger Spin and many of the more "tamer" rides, like Dumbo, Tomorrowland Speedway, and Mad Hatter Tea Party.
AK has Expedition Everest and Kali River Rapids - both are really fun, but be warned: on Kali, you will get wet, and most likely soaked! The safari ride, Lion King show and Finding Nemo show are really cool too.
And like Claudia said, you CAN bring in your own snacks and water. We do all the time and it saves us a ton of money. DD walks the whole time and gets hungry quite often, so it's good to have things like granola bars, PBJ sandwiches, Fig Newtons, bananas, etc. packed that you can just pull out of a back pack. Then it's ok in my mind to treat her to something like popcorn or a Mickey ice cream bar because we're not forking out wads of cash for snacks all the time. My stepsons were the same way when we took them when they were 6 and 7 - they walked a ton, never complained, but needed snacks constantly.
ETA: Depending on your child's endurance, it's usually really helpful to plan to take some kind of break in the middle of the day - head back to the hotel, swim in the pool, rest up, then head back in the evening for more fun.