You can go to guest relations at each park and they will give you a birthday pin from that park to wear on his clothes. I ordered a t-shirt from disney, (the kind you can design yourself) and had it say something like "I'm celebrating my birthday at disney world". The characters usually notice the pins and say something, as well as the cm's.
We were staying at WL and I mentioned to them at check in and when we got back to our room, DD had balloons and card from Mickey. She was thrilled!
Good luck, have fun.