Anything purchased through Disney or through a florist/gift service type of place will be expensive. You can order Birthday cakes and such which are also very expensive ($75+....and waaay up into hundreds of dollars) unless you buy the generic cake. They cannot have it waiting in your room though I don't think, and I'm not sure if they have cakes available at the Value resorts.
I like the idea of bringing a pack of balloons, maybe a few streamers or a birthday banner. When you arrive, have one of you check in/go up to the room and quickly decorate while the other lets the child explore the resort or gift shop for a few minutes...then have them text you when it's safe to go to the room and have the other say something like, "Mickey must have known it was your birthday!". If he is easily distracted (ie: more than 10-20 minutes), before having one of you go up to decorate the room/bring luggage in, they can stop at the food court or wherever they may sell a cupcake or some sort of dessert or treat (or even stuffed toy) he would like.