Hi Ginnett,
I'm so excited to hear that you're going to be visiting Walt Disney World! There's nothing better than seeing it through the eyes of your children...it is truly a magical experience!
I would definitely suggest that you take your son to
Magic Kingdom. It's my favorite, and it is wonderfully suited for kids his age, as well as adults. Not only will he have the chance to see and meet many of the loved Disney characters, he'll be able to enjoy so many of the attractions! Please take a look at Disney's fabulous
Planning for Pre-School Kids page. It's full of so many tips, and even gives you ideas for sample itineraries. The page also lists all of the attractions that are suitable for kids his age.
I promise you that Magic Kingdom is the best choice for you. You'll all enjoy it very, very much and the memories you'll be making will really last you a lifetime. My children are 18 and 15 years old and we STILL talk about the trips to Disney when they were little.
If you have any other questions please don't hesitate to come back and ask. We're always happy to help. Wishing you a very pixie dusted trip!