I hope that your DS has a very magical birthday!
Make sure that you tell everyone it his birthday. When you make res. hotel, dinner, etc. If they don't give him a button when you check in ask for one. He can get one for his birthday and a first trip button too. If you make dinner res. remind them when you check in at the podium.
Here are some ideas:
Free or little cost:
If you plan to see characters take an extra empty 9 year old Bday card. The characters can sign it and your ds will have it forever. Make him a birthday shirt wear. Often at dinner your DS will get a birthday cupcake (at sit down meals)
If he is a Star Wars fan and you plan to go to Hollywood Studios. Get to HS before the park opens. Ask at the gate where to sign up for the Jedi Academy. Get a map to know your way. (not sure of directions with the hat being removed) Go down the main street by the shops bare to the left go by the Hollywood and Vine rest. past the big green dinosaur, past Ind. Jones. THe place to sign up should be straight ahead. Sign up for a time to fight Darth Vader on stage. (our DS loved this -huge Star Wars fan)
Cost from home : Wrap small Disney themed presents and give them to every hour or every couple of hours (the dollar store had a lot)
Bring decorations Disney themed and decorate the room after he goes to sleep so he wakes up to it.
Cost:
Pirates Leg. in MK -It is awesome to begin with but they make a really big deal over you on your birthday.
Cake- can range from $20 ish to very expensive - There is a cake hotline phone to call in advance (like 2 weeks) for the very fancy fond. cakes. They are AMAZING!!! (and yummy too

) - either in your room or sit down dinner
If you plan to do a character meal do one on his birthday.
Fancy cake info :Specialty Cakes From Disney Website
Dining Extras | Special Experiences
A custom cake can be the icing on your celebration. Choose from a variety of cakes and shapes, personalized for your special occasion. Just give us 72 hours advance notice so we can make your customized creation. Available at most table-service restaurants. To order any custom bakery cake, call the Walt Disney World Resort at (407) 827-2253. Requires advanced reservations
Room decorations or basket from the Disney Florist (we have not done this)
Hotels:
Our DS loved the Carb. Beach when he was your DS age. Our kids like Art of Animations and Fort Wilderness.
No matter what you do, it will be a birthday he will never forget. Enjoy!