Does he like macaroni & cheese? There's lots of that around. You could always ask for a grilled cheese sandwich as well- I know at McDonald's they'll make a beefless cheeseburger for vegetarians, and I don't see why they couldn't do that at Disney too. There are also a lot of places that serve flatbread pizza.
The adults could also share their meatless options with him, like salad or plain pasta (if available).
What does he eat at home? You might want to pack some options for him when he's sick of PB&J.