The
travel agent may have been referring to the Birthday Promotion that was going on last year that is no longer going on.
Or... If you go under the "My Vacation" tab at the top and enter your reservation number and info, you'll see your itinerary and there's an "add a celebration" section. You can enter in your son's birthday. When we went last year for my boyfriend's first visit, he received his "first visit" button at check in. We also received a special Disney Photopass card that had a 20% off code on it.
Have a great trip!