Hi Tanya:
I have a five-year old and soon to be four-year old. Several months ago, we took both boys on Test Track and Big Thunder Mountain Railroad, and they LOVED both rides. However, both rides go very fast at points, so you really have to know your five-year old. I'd say of the two, see how your son does on Test Track first, since that ride is more tame and doesn't really speed up until the end.
As for other must-rides, I would love to share some of my family's must-dos for our Disney World vacations. Your family may have different tastes, but hopefully these recommendations will be a good start.
Among many other things: at the Magic Kingdom, we always ride the Jungle Cruise, Big Thunder Mountain Railroad, Peter Pan's Flight, Mickey's PhilharMagic, and it's a small world. At Epcot, we take in Spaceship Earth, Test Track, Living with the Land, Soarin', and tour the countries of World Showcase. Our must-dos at Hollywood Studios include Toy Story Mania and Muppet Vision 3-D. At Animal Kingdom, our family always rides the Kilimanjaro Safari and visits Conservation Station.
Enjoy your vacation.