Hi, Christy! We took our son to Disney World for the first time when he was three and a half and he loved it.
If you visit the
Traveling with Little Ones page on the Disney World Web site, you'll find all sorts of helpful planning tools, including a link on the right hand side for attraction height requirements. I think you'll be surprised with how many attractions have no height requirement at all. In fact, nothing in Fantasyland in the Magic Kingdom has a height requirement.
That being said, I'd recommend you start with some of the more easy-going rides to help your son become acclimated to the type of experience he can expect from attractions. For example, there are several attractions that use boats to move guests through the experience. He'll feel much more prepared for Pirates of the Caribbean or Maelstrom, for example, if you take him on it's a small world, Living with the Land or Gran Fiesta Tour first. Taking similar precautions can also help with other experiences. For dark rides, start with Peter Pan's Flight or The Many Adventures of Winnie the Pooh before considering Snow White's Scary Adventures or the Haunted Mansion. For coasters, if he's tall enough, try out the Barnstormer at Goofy's Wiseacre Farm before Big Thunder Mountain.
In terms of the best parks to visit each day, check out the crowd calendar on TouringPlans.com to get the most out of your days.
You're an awesome mom for bringing your little one to Disney World!