Hi there, Dana! Welcome to the Disney Parks Moms Panel - and happy early birthday to your big boy!
It's so much fun to experience Walt Disney World with kids. As my son gets a little older, he discovers new things all the time. I love his curiosity and his amazement when he finds something he hadn't seen before.
Little ones under 3 can enter the Theme Parks for free, and can share from their parents' plates at no charge in Walt Disney World restaurants - a big help for families who are trying to watch their budgets. In my experience, you should have no issues entering the parks with your almost-birthday boy, even if he's tall for his age. If you'd feel more comfortable taking a copy of his birth certificate you can certainly do that (leave the original at home), but I honestly don't think it's necessary. Even if a Cast Member asks you how old he is, they'll take your word for it.
While you're there, go ahead and pick up a free "Happy Birthday" button for your son. He can wear it during your vacation, or you can save it for his actual birthday when you get home. It will be a nice reminder of your trip and something special he can wear on his big day.
I hope this helps you plan your son's pre-birthday celebration, Dana. Please come visit us again if you have any more questions.
Have a magical day!
Kathy K.