Hi, Michelle!
The most economical way to travel to WDW is to stay in a Standard Room at one of the very fun Value Resorts. By booking through one of the
Special Offers you might get a discounted room or other available promotion. When adding on your park tickets, do not add on "options" such as park hopping or water parks, as this will really increase the cost of your tickets.
Plan to bring snacks for a quick morning breakfast, such as granola bars or cereal. Counter service meals are a great value, and often are big enough to share. Plan for one special meal at a table service restaurant, maybe Sci-Fi Dine-In Theater in Hollywood Studios or Whispering Canyon Cafe in Wilderness Lodge and let your server know you are celebrating a birthday. They will often bring out a cupcake or other fun dessert to celebrate.
When booking your reservation, you can add on your birthday celebration. While nothing special is guaranteed, you will likely receive a "Happy Birthday" button at check-in which your son can proudly wear during your vacation. Feel free to bring decorations for your room or a sign for your door to make things festive. You can also add your celebration onto any dining reservations that you make. Cakes are available for purchase at any table service restaurant, just let them know when checking in at the podium that you would like to buy a cake for the birthday boy!
Wishing your son a very magical 13th birthday celebration in Walt Disney World!