Hi Erica,
If your son is younger than 3 on the first day of your
Walt Disney World vacation, he will not need a ticket for admission, even if he turns 3 during your stay. If you have a copy of his birth certificate handy, it won't hurt to have it along, but I have never been asked for proof of age for young children, either my own or visiting relatives.
My older daughter was 2 1/2 on her first visit to the Magic Kingdom, and she says that trip is her first memory. I hope you and your families make some magical memories as well. Here's a link to some of the tips for visiting with
preschoolers and young children to help with your planning.
Thanks for visiting the Disney Parks Moms Panel, and please let us know if we can answer any other questions for you before you arrive. Enjoy!
Wilma