I am not very good with the "SURPRISE" Your going to Disney World part, however, I can come up with a 100 things to make or buy to use before, after and during the trip. LOL
My son's birthday is also in May and we are taking him to SWW the day after for his birthday. He already knows about the trip because we have been trying to do this for the past 3 years. On his birthday (the day before we leave) we are having Star Wars themed cupcakes with family and I am giving him a box filled with the following items:
SW Shirts to wear on the trip
SW Autograph Book that looks like Darth Vader
SW Movies on his iPad to watch on the way
SW Bucket (its the R2D2 popcorn bucket we got a few years ago but he wasn't with us and doesn't know we have it) filled with SW Snacks for the trip
SW Activity Book to help pass the time away (jtanddolly has a lot of SW Printable Games to fill the book with)
Then on the morning we leave I am going to have a Stuffed Chewbacca in his seat (I know he is too old for this but I just couldn't pass it up when I saw it.

) and I have three Star Wars Car Magnets I'm going to place on my car.
When we arrive at the hotel I am having a Gift Basket delivered to the room with the following in it:
SW Magic Band Accessories
SW Lanyard with Trading Pins
SW Action Figure
and the Star Tours Adventure Pack
I am making Pillow Treats to leave on his bed during the trip.
We are spending all our time at SWW so I have booked all the SWW Character Meals, the VIP Package, and one night we are doing dinner at Mama Melrose for his birthday dinner (his pick). I am having a Star Wars Jedi Mickey Cake delivered along with SW Balloons as a surprise during dinner.
Maybe this will help! Sorry I couldn't help with the BIG Reveal. I'm not good at keeping a secret.