Hi Angela,
I think you'll find that your son can ride most attractions. If you look at the
Attractions at Walt Disney World on the website, you can filter the attractions according to height. Select the
"Any Height" option to see all of the attractions that you and your family can enjoy together. Dumbo, Many Adventures of Winnie the Pooh, and Peter Pan's Flight are most certainly all on that list!
Having said that, some attractions are unappealing to small children, especially at 21 months. They start to be afraid of dark places or attractions with loud noises. You know your child best. Maybe he's still unaffected by these things, or maybe he finds them funny/silly. It's up to you to use your own discretion. Again, using the Attractions feature on the website you can filter
Recommended Ages. Just select
"Pre-schoolers" for a list of the best attractions for your child. I can tell you that some of the attractions we steered clear of at this age are: Honey, I Shrunk the Audience, Haunted Mansion (although it's pretty tame), It's Tough to Be a Bug, and Sounds Dangerous!. Pirates of the Caribbean is another favorite attraction that can have some scary elements.
Hope this helps. Have a magical vacation!