Hi Shannon! Sounds like a great birthday celebration for your special little guy!
You can order tee shirts from the Disney Store's website, found
HERE. If you click on the tab, "Disney Parks Products", you may find something special that fits the bill.
If you are interested in a customized tee-shirt, you can visit Disney's Design-A-Tee shop at Downtown Disney Marketplace in Walt Disney World. You can read more about this experience
HERE.
I also recommend that you visit Disney's webpage
HERE to discover many different options you have to make celebrations more special at Walt Disney World. You can choose pirate gifts, customized cakes, and personalized Birthday Ear Hats, just to name a few!
Chef Mickey and O'Hana are wonderful choices for celebrating, and I am sure you will be happy with these dining experiences. Be sure to inform the Cast Member serving you that you are celebrating a special birthday!
And, make sure to request a special celebration button for your son upon check-in at your resort or upon entering a park. He will feel very special as he receives special wishes all day long from Cast Members, and even guests, throughout the parks!
I hope you all have a wonderful time, and that he has a very Happy Birthday!