Universal has a birthday sticker that they will give to him to wear. Instead of doing this for my DS's birthday, I bought a spiderman birthday button on ebay for a few dollars that was customized with his name and his age.
If you are staying onsite, you should note his birthday on your reservation and let them know that you're celebrating at check in. We did this and they sent our DS 2 desserts with a birthday card from the front desk staff. (Not expected, but well-received.) We also had a birthday cake delivered to our room on his birthday so we could celebrate with cake and candles. This cost about $50 for a small cake, marble with raspberried in the center and whipped cream frosting. YUM!