Hi Brittany,
Welcome to the Disney Parks Moms Panel and thank you for your question!
As you undoubtedly know, Guests of the Walt Disney World Resort under the age of 3 are Mickey's Special Guests and don't require a ticket in order to enter a Theme Park or Water Park. My "little" guy is 2 1/2 years old and big for his age too, so I understand your concern. To give you an idea - he's tall enough to ride almost all of the attractions in the Theme Parks, even those with a
height requirement, but he's not "emotionally" ready to ride very many of them...
I always walk around with a photocopy of the front page of his passport, just in case people ask to see it. Although I routinely get asked for his age, no one has asked me to provide them with proof as of yet. I would recommend that you do the same: bring a photocopy of his birth certificate with you, so that you can have one less thing to worry about during your visit!
I hope my answer helps you plan your upcoming visit to Walt Disney World Resort! Please don't hesitate to come back and see us if you have any other questions.
Have a Magical Day,
Lynne