Hi Joan,
Epcot is my favorite theme park at Walt Disney World, and if you ask my 8-year-old daughter, she will tell you that it is hers as well - and it has been for her whole life. I think that you can spend a fun, full day at Epcot with a 6-year-old.
Starting in the morning in Future World, if your son is adventurous, he is likely tall enough to ride both Soarin' and Test Track. These amazing rides provide some thrill, but my daughter had conquered them both - and enjoyed them heartily - at 6 years old. For slightly less thrilling, but nonetheless very enjoyable, attractions, consider Spaceship Earth, The Seas with Nemo and Friends, Living with the Land, and Journey into the Imagination with Figment. My daughter really enjoys all of these rides, which are gentle and fun.
As the day goes on and you can move into World Showcase, your son might enjoy
Disney Phineas and Ferb: Agent P's World Showcase Adventure. Based on Disney's Phineas and Ferb, this interactive adventure lets children help catch Dr. Doofenshmirtz and foil his evil plans. Children are given a device which receives instructions at various locations in the World Showcase. It is a lot of fun!
Finally, one way that my family likes to enjoy World Showcase is by taking advantage of the Kidcot Fun Stops. At these stops, children can color a Perry the Platypus or Duffy Bear, as well as collect a stamp from each country. We like to wander the World Showcase grabbing a bite to eat and sitting down as our daughter colors at the Kidcot Fun Stops.
I hope that this answer gives you some ideas on how to spend a fun day at Epcot with your 6-year-old son, Joan. Please do not hesitate to ask if you have any further questions.