Hi Samantha! Welcome to planDisney! I’m glad you brought your question to us! Happy birthday to your son! I’m happy to clear up the confusion regarding admission for children who turn three while on their vacation.
Guests who are two and under do not need a ticket for admission to the Disney Parks! However, once a child turns three, they need a ticket to enter the Disney Parks. That is the case even if a child turns three during their vacation.
Samantha, keep in mind that it would be unusual for a Cast Member to question the age of a Guest, especially one so young. Children visiting the parks are not required to bring identification or a birth certificate. So, I guess it's best to say that Guests are on the "honor system" when it comes to ticket purchases for those Guests turning three during a vacation.
Your little Mouseketeer is going to have so much fun! Be sure to check out perfect attractions such as
Mickey and Minnie’s Runaway Railway,
Mickey’s PhilharMagic,
Festival of the Lion King, Peter Pan’s Flight, Buzz Lightyear’s Space Ranger Spin, Toy Story Mania,
Under the Sea ~ Journey of the Little Mermaid, and so much more!
Dreams do come true!
Jonathan