I'm not familiar with your area, but what I can tell you is think outside the box.
Today we had a 3rd birthday party at a local Children Museum, filled with all sorts of hands-on displays for kids. You can set up a party with the museum and pay, or do as I did and have lunch and cake out on the picnic tables on the side, then go into the museum. Everyone loved it, the kids all had a blast, we got great pictures of the kids on a real bus, wearing different costumes, performing on-stage, and lots more.
Look into different events scheduled in your area, too, ask your son what he would like to do, you may find something fun he'd love. Maybe some sort of sports event? My cousin is tying her son's birthday in with the railway celebration her town has every year, meeting the kids and their parents there, touring the trains and exhibits, then enjoying the huge barbecue set up for everyone and having cake there. Some of her party favors are wooden train whistles, and bandanna scarves, blue for the boys, pink for the girls.