My thoughts would be:
1) Take some glow necklaces with you, and have him give them out to other people so it feels like others are joining in his celebration. Or, if you're not comfortable with that, have everybody in your group wear them after dark.
2) Cupcakes! There are several threads on the Restaurant Board which show them. You can find them in every park. Easier than planning ahead and bothering with a big cake.
3) You can get a balloon in the Magic Kingdom, unless you will be staying at Animal Kingdom Lodge & Villas. Balloons are not allowed there, so buying a balloon which you can't have in the hotel would be a bad idea.