Hello Karen! Welcome to the Disney Parks Moms Panel.
I remember booking a last-minute Walt Disney World Resort vacation a couple of weeks
before my son turned 3 to take advantage of complimentary Theme Park Admission as one last hurrah of savings.
As long as your child turns 3 on a date after you have already Checked-In at your Walt Disney World Resort Hotel or activated a
Multi-Day Theme Park Admission Ticket for an accompanying Adult, then your little Mouseketeer can still be considered under 3 for the duration of your stay or length of said Theme Park Admission.
You should not be concerned about carrying proof of his age if you are
staying at a Disney Resort Hotel and have a
My Disney Experience Account as this information will already be linked to your MagicBands. However, if you are not staying at Disney Resort Hotel, it may be in your best interest to carry proof of age as you noted that your son is big for his age.
We were never questioned about proof of age when my kids were under 3, however, my little ones are on the smaller side.
Be sure to pick up a Happy Birthday Celebratory button from Guest Services when you arrive on your son's birthday.
Have a Magical Visit!
~Todd