J.P.
What about an Astronaut party. Have it in the back yard. Get a bunch of refrigerator boxes from Sears or Home Depot, and some other cardboard boxes of different sizes. A few rolls of duct tape, aluminum foil, scotch tape, markers, paints and invite over the class to build a space shuttle or space suits or whatever they want. They will come because, after all, how many astronaut parties have you seen? They can all have fun playing and inventing, but at the same time, Tyler can just work on his part without having to be super social, but still be part of the party. Most of the kids will want to do their own thing with it anyway, who doesn't like to invent! If he crawls inside a box to be alone, you can give him a marker or two to decorate the inside. All space ships need knobs and buttons!
It's inexpensive, inventive, low key, most of the kids will be entertained just with the building of things, and if a lot of kids don't come, the boxes are still big enough for mom and dad to play too. After the party is over, he can keep the ship (provided it doesn't rain) for as long as it will last.
My kids LOVE this!